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A 


PRACTICAL  TREATISE 

ON 

ELECTRIC  LIGHTING. 


. 


/ 


/ 


Plate  I. — map  showing  street  mains. 


ENGINEERING  DEPARTMENT  LIBRARY 
WESTERN  ELECTRIC  COMPANY 

A 

PRACTICAL  TREATISE 


ELECTKIC  LIGHTING 


J.  E.  H.  GORDON,  B.A.,  M.S.T.E. 

"Member  of  the  Paris  Congress  of  Electricians,  1881  ; 


MANAGER  OF  THE  ELECTRIC  LIGHT  DEPARTMENT  OF  THE  TELEGRAPH  CONSTRUCTION 
AND  MAINTENANCE  COMPANY. 


D.  APPLETON  AND  CO. 
1884. 


BY  THE  SAME  AUTHOR. 


Tii'o  volumes,  8 vo,  700  pp.,  with  very  numerous  coloured  and  full-page 
Plates , and  Illustrations  in  the  text , price  Seven  Dollars. 

A PHYSICAL  TREATISE  ON 

ELECTRICITY  AND  MAGNETISM. 


New  Yobk  : D.  APPLETON  & CO. 


PREFACE. 


When  the  Paris  Exhibition  of  1881  first  showed  to  the 
public  the  possibilities  of  electric  lighting,  it  was  assumed 
that  in  the  course  of  a few  months  the  electric  light  would 
be  universally  adopted.  That  assumption  has  proved  to  be 
unfounded,  and  the  public  have,  speaking  generally,  gone 
into  the  opposite  extreme,  and  declared  that  the  electric 
light  is  a failure. 

In  October,  1881,  I wrote  in  the  Quarterly  Review , “ The 
day  will  come  when  gaslight  will  be  as  obsolete  as  wooden 
torches,  and  when  in  every  house  the  incandescent  lamp 
will  have  replaced  the  gas-jet.” 

In  this,  the  darkest  hour  of  electric  lighting,  when  failure 
after  failure  has  occurred,  when  company  after  company  has 
become  bankrupt,  I unhesitatingly  repeat  this — I will  not 
say  opinion  but — conviction,  only  adding  to  it  my  further 
conviction  that  the  day  of  universal  electric  lighting  is  not 
even  in  the  near  future,  but  in  the  immediate  future. 

I reassert  this  conviction  the  more  confidently  because, 
having  been  myself  one  of  those  engaged  in  the  study  of 
electric  lighting  during  the  last  three  years  of  struggle  and 
labour,  I have  seen  not  only  the  cause  of  past  failures 
(failures  already  forgotten),  but  have  further  seen  the  steady 
growth  of  success,  as  invention  after  invention  has  stood  the 
test,  not  only  of  experiment,  but  of  months  of  careful  trial. 

The  only  cause  of  delay  which  I will  here  allude  to  is  the 
delay  in  the  commencement  of  public  works,  which  has  been 
undoubtedly  caused  by  the  action  of  the  Board  of  Trade. 

This  delay,  however  much  complained  of  by  the  public, 

oENGf&ERIflO  ET/.r? 

SESlEP.fi  EfICL:.  UlO  . 


lias  been  the  salvation  of  electric  lighting.  If  two  years 
ago,  as  in  the  case  of  some  American  cities,  our  streets  had 
been  allowed  to  be  opened  for  the  purpose  of  laying  electric- 
light  wires  by  every  speculator  or  inventor  who  thought,  or 
wished  the  public  to  think,  that  he  could  supply  electric 
light,  it  is  certain  that  by  now  the  number  of  accidents  and 
failures  which  would  have  occurred  would  have  ensured  the 
enactment  of  restrictions  far  more  stringent  than  any  con- 
tained in  the  present  law.  The  delay  caused  by  the  Board 
of  Trade  has  been  the  delay  not  of  the  dilatory  builder,  but 
of  the  wise  architect,  who  will  not  allow  the  build  ing  to  be 
commenced  till  he  has  seen  that  the  foundations  are  dug 
deep,  and  well  filled  with  sound  concrete. 

This  book  has  been  in  preparation  for  some  two  years,  and 
has  been  modified  again  and  again  as  the  science  of  which 
it  treats  has  progressed,  in  order  that  it  might  indicate  the 
state  of  that  science  very  nearly  up  to  the  present  date. 

In  describing  machines  and  lamps,  I have  not  thought  it 
necessary  to  describe  many,  but  have  selected  those  which 
are  typical  of  different  classes.  Further,  I have  described 
but  few  things  which,  however  useful  they  have  been  in  the 
past,  are  not  in  my  opinion  likely  to  be  useful  in  the  future. 

There  is  one  point  in  the  book  itself  to  which  I wish 
to  allude  : and  that  is  the  " method  of  calculating  the  horse- 
power wasted  in  street  mains,”  described  at  page  26.  It 
must  be  understood  that  the  method  there  given  is  only 
what  it  professes  to  be — the  method  of  calculation.  I do 
not  imply  that  the  method  of  arrangement  there  given  is  the 
one  which  I recommend. 

I have  to  thank  Mr.  Swan  for  much  information  as  to  the 
process  of  manufacture  of  his  incandescent  lamps,  and  for 
drawings  illustrating  it. 

I have  also  to  thank  Mr.  Crompton  for  accounts  of  his  arc 
lamps,  and  for  the  chapter  on  “ Carbons  for  Arc  Lighting/' 


28,  COLLINGHAM  PLACE,  LONDON,  S.W. 
April  29th,  1884. 


CONTENTS. 


CHAPTER  I. 

INTRODUCTORY. 

PAGE 

Principles  of  Artificial  Lighting 1 

CHAPTER  II. 

OX  THE  CONVERSION  OF  ELECTRIC  CURRENTS  INTO  HEAT. 

Analogy  between  Flow  of  Electricity  and  Flow  of  Water  . . 4 

Principles  of  Electric  Lighting  .......  7 

Amount  of  Light  produced  by  a given  quantity  of  Heat  depends  on 

Temperature  ..........  8 

Incandescent  Lamps  .........  8 

Arc  Lamps  ...........  9 

CHAPTER  III. 

ELECTRICAL  UNITS  AND  THEIR  RELATION  TO  EACH  OTHER,  AND  THE 
HEAT  AND  WORK  UNITS. 

Current— -Ampere  . . 10 

Pressure — Yolt 10 

Resistance — Ohm 11 

Quantity — Coulomb  .........  12 

Ohm’s  Law  ...........  12 

Units  of  Heat  and  Work,  and  their  Relation  to  the  Electrical  Units  13 

Energy  and  Horse-power 13 

Calculations — Relations  between  Horse-power,  Current,  and 

Resistance 14- 

Relations  between  Horse-power,  Current,  and  E.M.F.  . . 15 

Relations  between  Work,  Quantity,  and  E.M.F.  ...  17 

Summary  of  Formulae  . . 19 

The  Commercial  Electrical  Unit 20 

Value  in  Horse-power  per  Hour  ......  20 


X 


Contents . 


PAGE 

Value  in  Swan  Lamps  per  Hour 20 

Equivalent  in  Gas  .........  21 

Rule  for  Comparison  of  Prices  ......  21 

CHAPTER  IV. 

EULES  FOE  THE  EESISTANCES  OF  DIVIDED  CIECUITS. 

Law  of  Divided  Circuits 22 

Quantity  and  Series 23 

Shunts  ............  25 

Self -adjustment  of  Current  by  Lamps  in  Quantity  ....  26 

Method  of  calculating  H.P.  wasted  in  a Network  of  Conductors  . 26 

CHAPTER  V. 

EXPEEIMENTAL  MEASUEEMENT  OF  CUEEENT — ELECTEO-MOTIVE  FOECE 
— EESI8TANCE — H.P.  DEVELOPED  IN  EESISTANCE. 

Measurement  of  Current  by  the  Siemens  Electro-dynamometer  . 35 

Galvanometers  ..........  37 

Thomson’s  Reflecting  Galvanometer 38 

Galvanometer  Shunts  ........  39 

The  Tangent  Galvanometer 40 

Graduation  of  Tangent  Galvanometer  . . ...  41 

Ayrton  and  Perry’s  Ammeter  .......  44 

Thomson’s  Graded  Galvanometer  ......  47 

Measurement  of  Electro-motive  Force  by  the  Voltmeter ...  49 

Ayrton  and  Perry’s  Voltmeter  ......  50 

Thomson’s  Voltmeter  ........  50 

Cardew’s  Voltmeter 52 

Measurement  of  Resistance  by  Wheatstone’s  Bridge  ...  52 

Resistance  with  Strong  Currents 55 

The  Ohmmeter 56 

Measurement  of  H.P.  expended  in  a Circuit 58 

Measurement  of  Alternating  Currents 60 

CHAPTER  VI. 

INCANDESCENT  LAMPS. 

General  Principle 61 

The  Carbon 62 

The  Terminals  ..........  62 

Hot  Exhaustion  ..........  62 

The  Swan  Lamp 63 

The  Filament ..........  63 

The  Exhaustion  .........  64 


Contents . 


xi 


PAGE 

Steam’s  Air-pump 65 

Mounting  . . . . . . . , .66 

Efficiency  ..........  66 

Current,  E.M.F.,  and  Copper 67 

The  New  Swan  Lamp 67 

Process  of  Manufacture  ........  68 

Efficiency  ..........  70 

The  Holder 70 

Special  Lamps  ..........  71 

Swan’s  Miner’s  Lamp 72 

The  Edison  Lamp 73 

Efficiency  ..........  74 

Efficiency  and  Durability  . . . . . . . .74 

Temperature  Scale  for  Incandescent  Lamps 74 

The  Maxim  Lamp  ..........  75 

The  Lane-Fox  Lamp 78 

The  Furnace 79 

The  Lane-Fox  Air-pump 82 


CHAPTER  VII. 

ARC  LAMPS. 


General  Principle  .......  . . 84 

The  Serrin  Lamp 86 

The  Crompton  Lamp  .........  87 

E Pattern  ..........  87 

K Pattern  ..........  89 

D.  D.  pattern  (the  present  form)  ......  91 

Adaptation  of  the  D.  D.  Lamp  to  Alternating  Currents  . . 94 

Mounting  ..........  95 

The  Brush  Lamp  ..........  95 

The  Jablochkoff  Candle 99 


CHAPTER  VIII. 

CARBONS  FOE  AEC  LAMPS. 


General  Principles  ..........  100 

Requirements  ..........  101 

Purity  ' 101 

Regular  Density 103 

Mechanical  Perfection  of  Form  103 

Electrical  Resistance  of  Carbon  Rods  ......  104 

The  Size  of  Carbon  Electrodes 105 

Experiments  on  various  Carbons 106 


Xll 


Contents. 


CHAPTER  IX. 

MAGNETS  AND  ELECTRO-MAGNETIC  INDUCTION. 

PAGE 


Magnets — Preliminary  Note 108 

Relation  between  Polarity  and  Direction  of  Current  . . . 108 

Electro-magnetic  Attractions  and  Repulsions  . . . . 109 

Lines  of  Magnetic  Force 109 

Magnetic  Field  110 

Magnetic  Induction . . 110 

Experimental  Tracing  of  the  Lines  of  Force  of  a Magnet  . . 110 

Continuity  of  Physical  Change  .......  Ill 

Electro-magnetic  Induction  ........  Ill 

Theory  of  Electric  Generators  113 

Lenz’s  Law  as  to  Direction  of  Induced  Currents  ....  115 

Induction  by  variation  of  Current  in  one  of  two  Circuits  . . 115 

Iron  Core  ...........  116 

Effect  of  Iron  Core  on  the  Coil  surrounding  it.  . . . .116 

Moving  Magnet  ..........  116 

Construction  of  Electro- magnets  and  Measurement  of  Magnetic 

Fields 117 

General  Rule  . 117 

Saturation  ..........  117 

Measurement  of  Magnetic  Field 117 

Gordon’s  Field  Measurer • . . 117 

Apparatus  for  use  in  confined  Spaces 120 

Comparative  Measures  of  the  Coefficients  of  Mutual  Induction  of 

various  shaped  Coils  and  Electro-magnets  ....  121 

Self-Induction  ..........  122 

Coefficient  of  Self-Induction 125 

Coil  of  Maximum  Self-Induction 126 

Comparison  of  the  Coefficients  of  Self-Induction  of  Two  Coils  . 127 
Practical  Method  ......  128 


CHAPTER  X. 

General  Principles  of  Electrical  Generators 

Efficiency  of  Machines 

General  Types 

Motion  of  a Magnet  past  a Coil  with  an  Iron  Core 
Phases  of  an  Alternating-current  Machine 
Reaction  on  the  Magnets  .... 

Effect  of  Self-Induction 

General  Principle  of  Direct-current  Machines  . 

The  Gramme  Sub-type  .... 

The  Collector 

The  Siemens  Sub -type  .... 


130 

130 

131 

132 

134 

135 
137 

137 

138 
140 
140 


Contents . xiii 

PAGE 

Production  of  Magnetism  in  the  Field  Magnets  ....  141 

Series,  Shunt,  and  Compound  Winding 142 

CHAPTER  XI. 

ON  DESIGNING  DYNAMOS  AND  ON  THEIR  MECHANICAL  CONSTRUCTION. 

Application  of  Mathematical  Analysis  to  Machine  Construction  . 143 

General  Methods  of  Designing 145 

Calculation  of  Diameter  of  Wire  . 145 

Armature  Coils  . . . 148 

Mechanical  supporting  of  Coils  .......  149 

Centrifugal  Force 150 

Horse-power  Pull  ..........  151 

Factor  of  Safety , ....  152 

Strength  of  Materials  .........  152 

Use  of  Factor  of  Safety  in  Designing 153 

Speed 153 

CHAPTER  XII. 

SOME  TYPICAL  ALTERNATE-CURRENT  MACHINES. 

The  De  Meritens  Magneto  Machine  ......  156 

The  Siemens  Alternating  Machine 159 

The  Ferranti  Machine 160 

The  Gordon  Dynamo  Machine  .......  162 

CHAPTER  XIII. 

SOME  TYPICAL  DIRECT-CURRENT  MACHINES. 

The  Gramme  Sub-type . .167 

The  Crompton-Burgin  Machine 167 

The  Collector  . . . . . . . .167 

The  Brush  Machine  ........  169 

The  Siemens  Sub-type  .........  174 

The  Siemens  Machine  ........  174 

The  Edison  Machine  ........  176 

CHAPTER  XIV. 

Regulation  of  Machines .........  178 

Regulation  of  the  Gordon  Machine 179 

Automatic  Electric  Governors  .......  180 

The  Willans  Governor  ........  180 

Compound  Winding 182 

Conclusion  ...........  183 


xiy 


Contents. 


CHAPTER  XV. 

PAGE 

On  the  proposed  Distribution  of  Electricity  by  Secondary  Generators  184 

The  Goulard  and  Gibbs  System 184 

Efficiency 185 

Minimum  Useful  Efficiency 186 

CHAPTER  XYI. 

ON  THE  “ STORAGE  ” OF  ELECTRICITY — SECONDARY  BATTERIES. 

Storage  of  Energy 189 

Secondary  Batteries 190 

Storage  of  High-pressure  Currents 191 

CHAPTER  XVII. 

EXPERIMENTAL  MEASUREMENT  OF  HORSE-POWER. 

Horse-power  developed  by  an  Engine 193 

Indicators  ...........  193 

Richards’  Indicator  ........  194 

Boys’  Engine-Power-Meter  .......  195 

Maximum  Horse-power  of  an  Engine  ......  196 

Horse-Power  given  off  by  a Shaft  .......  196 

Speed — Young’s  Speed  Indicator 197 

CHAPTER  XVIII. 

Photometry 199 

Law  of  Inverse  Squares 199 

Graduation  of  Photometer  B oard 199 

CHAPTER  XIX. 

Central  Station  Lighting  ........  201 

CHAPTER  XX. 

METERS. 

Edison’s  Meter 202 

Hopkinson’s  Meter  .........  202 

Gordon’s  Meter 202 


Contents. 


xv 


CHAPTER  XXI. 

FIRE  RISKS. 

PAGE 

Rules  for  the  Prevention  of  Fire  Risks  drawn  up  by  the  Society  of 

Telegraph  Engineers  ........  203 

I.  The  Dynamo  Machine 204 

II.  The  Wires 205 

III.  Lamps  ..........  206 

Safety  Fuse  ...  206 


APPENDIX. 

TABLES. 

Weights  and  Resistances  of  pure  Copper  Wires  ....  208 

Natural  Sines,  Tangents,  &c 216 

Areas  of  Circles 217 

Strength  and  Weight  of  Metals 222 


LIST  OF  PLATES. 


PLATE  PAGE 


I.  Map  showing  Street  Mains  .... 

Frontispiece. 

II.  The  Siemens  Electro-dynamometer  . 

to  face  35 

III.  Thomson’s  Reflecting  Galvanometer 

. • 38 

IY.  The  Swan  Lamp — Old  Pattern 

. 63 

Y.  Steam’s  Air-pump 

. 65 

YI.  The  Swan  Lamp — New  Pattern 

. 67 

VII.  Furnace  used  in  Manufacture  of  Lane-Fox  Lamps 

. 79 

VIII.  The  Lane-Fox  Lamp 

. 80 

IX.  Lane-Fox’s  Air-pump  ..... 

. 82 

X.  Serrin’s  Arc  Lamp  ...... 

. 86 

XI.  Crompton’s  Arc  Lamp — Old  Pattern 

. 87 

XII.  Crompton’s  Arc  Lamp — New  Pattern 

. 91 

XIII.  Brush’s  Arc  Lamp  ...... 

XIY.  Lines  of  Magnetic  Force  ..... 

. 110 

XV.  Gordon’s  Magnetic  Field  Measurer  . 

. 118 

XYI.  Phases  of  an  Alternate  Current  Machine  . 

. 134 

XVII.  The  De  Meritens  Magneto  Machine  . 

. 158 

XVIII.  The  Gordon  Dynamo  Machine 

. 163 

XIX  ") 

£ The  Crompton-Burgin  Dynamo  Machine 

. 168,  169 

XXI.  The  Brush  Dynamo  Machine  .... 

. 170 

XXII.  The  Edison  Dynamo  Machine  .... 

. 176 

XXIII.  Apparatus  for  Regulating  the  Gordon  Dynamo 

. 179 

A 


PRACTICAL  TREATISE 


ON 

ELECTRIC  LIGHTING. 


CHAPTER  I. 

INTRODUCTORY. 

In  all  systems  of  artificial  lighting  of  whatever  kind,  the 
light  is  produced  by  the  incandescence  or  glowing  of  solid 
particles  of  matter.  The  heat  required  to  produce  this 
incandescence  is  produced  in  various  ways.  Our  ordinary 
coal-gas  consists  of  a combustible  gas,  richly  charged  with 
very  small  solid  particles  of  carbon,  which  being  made  white 
hot  by  the  combustion  of  the  gas,  glow  and  produce  the  light 
required. 

If  these  solid  particles  are  removed,  the  combustion  of  the 
gas  produces  no  light.  If,  for  instance,  we  mix  with  the 
gas  a sufficient  quantity  of  air  to  oxidise  and  consume  the 
carbon  particles,  we  obtain  a flame  hotter  than  the  ordi- 
nary gas  flame,  but  giving  no  light  at  all.  A burner  con- 
trived specially  to  mix  the  right  proportion  of  air  with  coal- 
gas,  and  known  as  the  “ Bunsen  burner/'’  is  much  used  for 
cooking,  and  for  other  purposes  where  heat  without  light  is 
required. 

Again,  if  we  burn  pure  hydrogen  gas  we  shall  obtain  a 
flame  giving  great  heat,  but  no  light,  because  the  hydrogen 
does  not  contain  any  solid  particles.  If,  however,  we  introduce 
a little  spiral  of  fine  platinum  wire  into  the  flame,  the  heat 
of  combustion  will  make  the  wire  white-hot,  and  it  will 
glow  and  give  light.  If,  instead  of  the  thin  platinum  wire. 


B 


2 


Electric  Lighting . 

we  use  a thick  one,  it  will  only  become  perhaps  just  red-hot, 
and  although  the  same  quantity  of  heat  is  being  used,  for  the 
same  quantity  of  hydrogen  is  being  burned,  yet  the  wire 
gives  very  much  less  light  than  before. 

If  now,  instead  of  allowing  the  hydrogen  to  burn  in  the 
air  where  the  oxygen,  with  which  it  is  combined,  being 
diluted  with  nitrogen,  is  diffused  over  a considerable  space, 
we  supply  it  with  pure  oxygen,  the  heat  produced  is  con- 
centrated in  a much  smaller  space,  and  the  temperature  of 
the  flame  is  consequently  much  higher.  If  the  mixed  gases 
are  supplied  under  pressure,  the  size  of  the  flame  is  still 
further  reduced,  and  it  can  be  concentrated  on  a very  small 
portion  of  the  surface  of  any  solid  body  against  which  it 
may  be  directed.  When  the  “ oxy-hydrogen  jet  ” is  directed 
against  a cylinder  of  lime,  it  raises  a portion  of  its  surface  to  a 
very  high  temperature  indeed,  and  the  heated  lime  gives  off 
an  intense  light.  This  arrangement  is  well  known  as  the 
" Lime-light.” 

Now,  in  all  these  arrangments  we  get  a certain  definite 
quantity  of  heat  from  a given  quantity  of  fuel  consumed, 
and  this  amount  is  the  same  in  whatever  way  the  combustion 
takes  place.  The  same  total  quantity  of  heat  is  produced  by 
the  combustion  of  a cubic  foot  of  hydrogen,  whether  it  burns 
with  a large  flame  in  air,  or  whether  it  burns  in  an  oxy- 
hydrogen  blow-pipe. 

The  amount  of  light,  however,  which  is  produced  by  the 
expenditure  of  a given  quantity  of  gas,  or  by  the  production 
of  a given  quantity  of  heat,  depends  entirely  on  the  way 
that  heat  is  applied. 

Let  us  consider,  for  instance,  the  heat  produced  by  the 
combustion  of  a cubic  foot  of  hydrogen  burnt  in  ten  minutes. 
This  heat  may  be  expended  in  boiling  a certain  quantity  of 
water,  in  which  case  it  produces  no  light  at  all ; or  it  may 
be  employed  in  heating  a thick  platinum  wire  to  dull  redness, 
in  which  case  it  produces  a little  light,  or  it  may  be  used  to 
heat  a thin  wire  to  whiteness,  producing  a considerable 
light ; or  finally,  by  means  of  the  oxy-hydrogen  jet,  it  may 
be  employed  in  heating  a piece  of  lime  to  intense  whiteness, 
and  produce  the  lime-light. 


o 


Principles  of  Artificial  Lighting. 

We  see,  therefore,  that  to  produce  a light,  we  must  heat  a 
solid  body  to  incandescence.  To  produce  a given  light  with 
the  smallest  possible  expenditure  of  heat  (that  is  of  fuel  and 
cost)  we  must  concentrate  our  heat  on  a solid  body  of  the 
smallest  possible  size , so  that  that  may  be  raised  to  the  highest 
possible  temperature. 

Solid  bodies  can  be  rendered  incandescent,  and  made  to 
give  light  by  heat  produced  otherwise  than  by  combustion. 
In  the  old  Flint-mill,”  used  by  miners  before  the  invention 
of  the  safety-lamp,  heat  was  produced  by  means  of  the 
energy  applied  by  the  man  who  turned  the  handle  and 
caused  a flint  and  steel  to  be  continually  knocked  together. 
The  heat  caused  the  incandescence  of  the  particles  of  flint 
knocked  off,  and  the  stream  of  sparks  gave  a certain  quantity 
of  light. 

Solid  bodies  can  also  be  made  hot  by  the  passage  of  a 
current  of  electricity  through  them.  It  will  be  our  object 
in  this  Treatise  to  discuss  the  various  methods  of  producing 
by  electricity  a temperature  in  solid  bodies  sufficient  to  cause 
them  to  give  light.  We  will  then  compare  the  temperatures 
to  which  the  incandescent  solids  are  brought  by  heat  produced 
by  combustion  as  in  a gas-flame,  and  by  heat  produced  by 
the  combustion  of  coals  in  a steam-engine  and  converted 
into  electricity  in  a dynamo  machine,  and  then  we  shall  be 
able  to  compare  the  relative  heat-economies  of  the  two 
systems  of  lighting.  To  determine  the  relative  money- 
economies,  we  must  further  discuss  the  various  expenses 
incidental  to  the  two  systems,  such  as  interest  on  plant, 
attendance,  loss  in  transmission,  &c.,  and  their  relative 
convenience  in  use. 

We  shall  commence  by  a discussion  of  the  principles 
involved  in  the  economic  production  of  electrical  energy,  and 
its  conversion  into  heat  and  light,  and  go  on  to  a descrip- 
tion of  some  of  the  best  and  most  typical  apparatus  now 
in  actual  use  for  the  purpose,  omitting  for  the  most  part 
descriptions  of  machines  and  lamps  which,  whatever  their 
merits  in  the  past,  are  not  likely  to  be  much  used  in  the 
future. 


A’ 


E lectric  L ighting . 


CHAPTER  II. 

ON  THE  CONVERSION  OF  ELECTRIC  CURRENTS  INTO  HEAT. 

(1.)  * Let  ns  suppose  we  have  a long  water-pipe  of  large 
bore,  bent  round  so  that  its  two  ends  dip  into  the  same 
cistern  at  the  same  level,  and  let  a force-pump  be  con- 
nected to  one  end.  We  see  that  by  a very  small  power  we 
can  cause  a stream  of  water  to  flow  round  it.  The  only  force 
opposing  the  motion  will  be  the  friction  of  the  water  in  the 
pipe.  To  overcome  this  friction,  however,  a certain  quantity 
of  heat  has  to  be  expended  in  the  steam-engine  working  the 
force-pump,  and,  by  the  friction,  the  sides  of  the  pipe  and 
the  water  will  be  more  or  less  heated. 

(2.)  If  the  pipe  is  of  small  bore  then  more  work  will  have 
to  be  expended  to  send  a given  stream  of  water  through  the 
pipe,  and  the  friction  being  greater  the  pipe  will  be  more 
warmed. 

(3.)  We  see,  then,  that  the  stream  of  water  has  given  us  a 
means  of  taking  heat  from  the  engine  fire,  and  conveying  it 
to  a distance,  namely,  to  every  portion  of  the  sides  of  the 
pipe. 

(4.)  As  long  as  the  pipe  is  of  the  same  bore  throughout, 
the  friction  will  be  the  same  at  all  parts,  and  the  heating  will 
be  uniform  all  along  the  pipe.  If,  however,  we  were  to  cut  our 
pipe  at  one  place,  and  interpose  a spiral  of  very  fine  tube,  a 
great  deal  of  friction  would  be  concentrated  at  one  spot,  and 
instead  of  the  heating  being  uniform  all  along  the  pipe,  by 
far  the  greater  portion  of  the  total  heating  would  take  place 

* Compare  the  numbered  paragraphs  with  those  having  corresponding 
numbers  on  page  6. 


Heat  produced  by  Friction  of  Water.  5 

in  the  spiral.  Thus  this  arrangement  of  the  stream  has 
given  ns  a means  of  taking  heat  from  the  engine  fire,  and 
conveying  it  to  any  one  place  we  like  at  a distance,  namely 
the  place  where  we  have  put  the  spiral. 

(5.)  In  both  cases  we  have  expended  mechanical  worh  in 
forcing  a current  of  water  through  a pipe  offering  resistance  to 
the  flow,  which  pipe  by  its  resistance  has  reconverted  a por- 
tion of  the  current  into  heat.  The  distribution  of  the  heat 
depends  on  the  distribution  of  the  resistance.  When  the 
resistance  is  evenly  distributed  all  along  the  pipe  the  pipe  is 
evenly  warmed.  When  the  greater  portion  of  the  resist- 
ance is  concentrated  at  one  spot  the  greater  portion  of  the 
heat  is  produced  at  that  spot.* 

(6).  We  must  particularly  note  that  the  heat  used  has 
been  expended  in  forcing  a current  of  water  through  a 
resistance,  and  not  in  producing  the  water  itself;  and 
that  if  a pipe  could  be  made  without  friction,  and  thus 
offering  no  resistance  to  the  flow,  then  that  a stream  of 
water,  however  strong,  when  once  started  would  go  on 
flowing  round  the  pipe  for  ever  without  the  expenditure  of 
any  work  at  all.f 

There  are  a class  of  substances  called  “ conductors/'  along 
which  a current  of  electricity  can  be  made  to  flow  in  the 
same  way  as  our  current  of  water  round  the  pipe.  Sub- 
stances along  which  electricity  will  not  flow  are  called 
insulators.  No  substances  are  quite  perfect  either  way,  the 
best  conductors  offer  some  resistance  to  the  flow,  and  the 
best  insulators  allow  a little  electricity  to  pass  through  them; 
but  for  our  present  purpose  the  metals  and  carbon  among 
solids,  and  intensely  heated  or  highly  rarefied  air  and  gases 
may  be  classified  as  conductors,  and  all  other  solids,  and  air 
and  gas  at  ordinary  temperatures  and  pressures,  may  be 
called  insulators.  With  the  conducting  and  insulating 
powers  of  liquids  we  have  at  present  nothing  to  do. 

Conductors  differ  greatly  among  themselves  in  the  facility 

* For  the  purpose  of  this  illustration  we  consider  the  heat  to  stay  in 
that  portion  of  the  pipe  in  which  it  is  produced,  and  not  to  be  carried 
away  by  the  water. 

f Newton,  Lex.  I. 


6 


Electric  Lighting. 

with  which  they  conduct  electricity.  A platinum  wire,  for 
instance,  offers  between  five  and  six  times  the  resistance  to 
the  flow  of  electricity  as  a copper  wire  of  the  same  length 
and  diameter.  Also  the  resistance  of  a given  length  of  a 
given  wire  is  greater  when  the  wire  is  thinner,  being  in- 
versely proportional  to  its  cross  section.  With  the  same 
cross  section  it  is  directly  proportional  to  the  length. 

(1.)  * Let  us  now  suppose  that  by  means  of  a steam- 
engine  turning  an  electric  generator  we  are  forcing  a 
current  of  electricity  through  a long  copper  wire  of  large 
diameter.  The  only  force  opposing  the  flow  will  be  the 
resistance  of  the  wire.  To  overcome  this  a certain  quantity 
of  heat  has  to  be  expended  in  the  steam-engine  working  the 
electric  generator,  and  by  the  resistance,  the  wire  will  be 
more  or  less  heated. 

(2.)  If  the  wire  is  of  smaller  diameter  the  resistance  will 
be  greater,  more  work  will  have  to  be  expended  to  send  a 
given  current  through  it,  and  more  heat  will  be  produced  in 
the  wire. 

The  relative  amounts  of  work  expended  and  heat  pro- 
duced in  sending  a current  of  electricity  through  a thick 
and  a thin  wire  of  the  same  length  and  material  are  inversely 
proportional  to  the  cross  sections  of  the  wires. 

(3.)  We  see  that  the  electric  current  has  given  us  a means 
of  taking  heat  from  the  steam-engine  and  conveying  it  to  a 
distance,  namely,  to  every  portion  of  the  wire. 

(4.)  As  long  as  the  wire  is  of  the  same  diameter  and  of 
the  same  material  throughout,  the  resistance  will  be  the 
same  in  all  parts,  and  the  heating  will  be  uniform  all  along 
the  wire.  If,  however,  we  cut  our  copper  wire  at  one 
place  and  interpose  a spiral  of  very  fine  platinum  wire,  a 
great  deal  of  resistance  will  be  concentrated  at  one  spot, 
and  instead  of  the  heating  being  uniform  all  along  the  wire, 
by  far  the  greater  portion  of  the  total  heating  will  take 
place  in  the  spiral. 

Thus  this  arrangement  of  the  electric  current  has  given  us 
a means  of  taking  heat  from  the  engine -fire  and  conveying 

* Compare  the  numbered  paragraphs  with  those  having  corresponding 
numbers  on  page  4. 


ENGINEERING.  DEPARTMENT  LIBBABf 
WESTERN  ELECTRIC  GOyPANY 

_/7<?rt7  produced  by  an  Electric  Current.  7 

it  to  any  one  place  we  like  at  a distance,  namely,  the  place 
where  we  have  put  the  spiral. 

(5.)  In  both  cases  we  have  expended  mechanical  work 
in  forcing  a current  of  electricity  through  a wire  offering 
resistance  to  the  flow ; which  wire,  by  its  resistance,  has 
reconverted  a portion  of  the  current  into  heat.  The  distri- 
bution of  the  heat  depends  on  the  distribution  of  the  resis- 
tance. When  the  resistance  is  evenly  distributed  all  along 
the  wire,  the  wire  is  evenly  warmed.  When  the  greater 
portion  of  the  resistance  is  concentrated  at  one  spot,  the 
greater  portion  of  the  heat  is  produced  at  that  spot. 

(6.)  We  must  particularly  note  that  the  heat  used  has 
been  expended  in  forcing  a current  of  electricity  through  a 
resistance,  and  not  in  producing  the  electricity  itself  (what- 
ever that  may  be) ; and  that  if  a wire  could  be  made  offering 
no  resistance,  then  a stream  of  electricity,  however  strong 
when  once  started,  would  go  on  flowing  round  the  wire  for 
ever  without  the  expenditure  of  any  work  at  all. 

An  electro-magnet  consists  of  a bar  of  soft  iron  surrounded 
by  a coil  of  copper  wire.  When  an  electric  current  is  sent 
round  the  wire  the  iron  bar  becomes  a magnet.  The  copper 
wire  offers  resistance  to  the  flow  and  becomes  heated,  and 
therefore  work  has  to  be  expended  at  the  generator  to  keep 
up  the  flow. 

In  a permanent  steel  magnet,  according  to  the  theory  of 
Ampere,  the  magnetism  is  produced  by  the  continuous 
flowing  of  electric  currents  in  channels  of  no  resistance 
which  surround  the  molecules. 

We  may  therefore  regard  a permanent  steel  magnet  as 
an  electro-magnet  surrounded  by  a wire  of  no  resistance,  in 
which  the  current  having  once  been  started  by  whatever 
process  of  magnetization  has  been  adopted,  continues  to 
flow  eternally  without  producing  any  heat  or  requiring  any 
heat  to  maintain  it. 

When  at  the  place  where  we  wish  to  concentrate  our 
heat,  we  place  a body  of  sufficiently  high  resistance,  and 
send  a sufficiently  strong  current  through  it,  we  can  make 
it  so  hot  that  it  will  glow  and  give  light . This  is  the 
principle  of  all  electric  lamps  of  whatever  kind. 


8 


E lectric  L igh  ting. 

We  have  stated  on  page  8 that  for  light  to  be  eco- 
nomically produced,  it  is  necessary  to  raise  the  body  pro- 
ducing the  light  to  the  highest  possible  temperature ; or,  in 
other  words,  to  concentrate  the  heat  in  a solid  of  the  smallest 
possible  size  or  with  the  smallest  possible  cooling  surface. 

Now  let  us  take  a platinum  spiral  such  that  a given  cur- 
rent makes  it  just  red  hot.  We  get  a very  little  light.  If 
we  take  another  spiral  composed  of  half  the  length  of  wire, 
and  whose  wire  has  half  the  section,  it  will  have  the  same 
resistance  as  the  first,  and  the  same  current  passing  through 
it,  will  produce  the  same  quantity  of  heat  in  it,  and  expend 
the  same  quantity  of  heat  in  the  steam  engine.  The 
platinum  having  very  much  smaller  cooling  surface  will  be 
raised  to  a much  higher  temperature,  and  will  become  white 
hot  and  give  a brilliant  light. 

If,  the  resistance  being  still  kept  constant,  the  surface  be 
further  reduced,  the  temperature  will  be  still  further  raised, 
and  the  same  amount  of  heat  will  produce  a still  more 
brilliant  light.  It  appears  then  as  if  by  making  the  wire 
still  thinner  we  could  produce  as  much  light  as  we  pleased 
from  a given  quantity  of  heat. 

The  practical  reason  why  we  cannot  do  this  is  that 
platinum  and  all  other  metals  fuse  at  what,  in  electric  light- 
ing, is  a comparatively  low  temperature.  Platinum  fuses  at 
about  2000°  C.  Further,  if  heated  in  air,  all  known  sub- 
stances rapidly  oxidise  and  burn  away. 

The  problem,  then,  which  has  had  to  be  solved  in  electric 
lighting  has  been  to  obtain  a substance  having  a resistance  of 
convenient  magnitude  which  can  be  heated  by  the  current 
and  which  is  either  indestructible  by  intense  heat  or  if 
slowly  destroyed  is  capable  of  easy  and  continuous  renewal. 

Carbon,  either  alone  or  in  conjunction  with  heated  air, 
satisfies  these  conditions  in  a great  measure.  It  has  never 
yet  been  fused,  and  though  it  slowly  oxidises  when  heated 
in  air,  yet  its  destruction  can  be  either  guarded  against  or 
compensated  for  as  is  done  respectively  in  the  two  great 
systems  of  electric  lighting  now  in  use,  i.e.  the  incandescent 
and  the  arc  system. 

In  the  “ Incandescent  ” lamps  of  Swan,  Edison,  Maxim, 


Arc  and  Incandescent  Lamps. 


9 


Lane-Fox,  Ac.,  the  resistance,  used  to  convert  the  current 
into  heat,  is  that  of  a very  fine  thread  or  wire  of  carbon 
which  is  brought  to  a state  of  intense  incandescence  by  the 
passage  of  a current  through  it,  and  which  is  protected  from 
oxidation  by  being  hermetically  enclosed  in  a glass  globe 
from  which  all  the  air  has  been  exhausted.  These  lamps 
last  for  many  months  at  a time,  if  not  too  much  heated.* 

In  the  “ arc  ” lamps  the  current  is  sent  through  two 
stout  rods  of  carbon  which  touch  each  other  end  to  end. 
As  soon  as  the  current  is  established  the  rods  are  separated  a 
little  way,  and  the  current  continues  through  the  heated  air, 
which  is  a partial  conductor  of  high  resistance.  Great- 
heat  is  produced,  the  “ poles  ” of  the  carbon  rods  glow  with 
an  intense  whiteness,  and  small  particles  of  carbon  becoming 
detached  are  heated  in  the  air  between,  and  form  a luminous 
“arc”  from  one  pole  to  the  other  which  adds  to  the  light. 

In  this  class  of  lamps  the  carbons  being  thicker  can  be 
raised  to  a much  higher  temperature  than  the  carbon  threads 
of  “ incandescent  ” lamps,  and  consequently  they  give  much 
more  light  for  a given  quantity  of  heat,  and  are  so  much 
the  “more  efficient.” 

The  carbon  rods  slowly  consume  away,  and  therefore 
have  to  be  fed  forward  by  suitable  machinery.  In  arc 
lamps  the  expense  of  the  carbon  rods  has  to  be  added 
to  the  cost  of  producing  the  current  in  estimating  the  total 
cost  of  the  light.  In  our  6th  and  7th  chapters  we  shall 
describe  various  lamps,  both  “ incandescent  31  and  “arc,” 
now  in  use,  but  as  the  lamps  have  to  be  constructed 
to  use  currents  of  certain  strengths,  and  as  each  lamp  is 
intended  to  convert  a certain  definite  quantity  of  electric 
energy  into  heat  it  is  necessary  for  electrical  engineers  to 
comprehend  the  methods  by  which  electrical  quantities  are 
measured  and  the  standards  to  which  they  are  referred. 
We  shall  therefore  devote  our  next  three  chapters  to  an 
account  of  the  standards  now  in  use,  and  the  methods  of 
measuring  electrical  quantities  by  means  of  them. 

* If  overheated  even  in  a perfect  vacuum  the  filaments  are  destroyed 
with  more  or  less  rapidity  by  some  process  analogous  to  mechanical  dis- 
integration—they  are,  as  it  were,  shaken  to  pieces. 


io 


Electric  Lighting. 


CHAPTER  III. 

ELECTRICAL  UNITS  AND  THEIR  RELATION  TO  EACH  OTHER,  AND 
TO  THE  HEAT  AND  WORK  UNITS. 

In  order  to  force  water  through  a pipe  offering  resistance 
to  the  flow,  a certain  pressure  or  “ aqua-motive  force 33  must 
be  supplied  by  a force-pump  or  otherwise.  Similarly,  in 
order  to  force  a current  of  electricity  through  a wire,  a 
certain  electric  pressure  or  electro-motive  force  must  be  sup- 
plied by  the  generator.  A small  electro-motive  force  will 
force  a small  current  through  a given  resistance  ; a larger, 
a proportionately  larger  one. 

Current. — Ampere. 

The  unit  of  electric  current  is  called  the  Ampere.  It 
is  a unit  of  flow  or  of  stream.  We  speak  of  an  electric  cur- 
rent of  so  many  amperes  in  the  same  way  as  we  might  speak 
of  a water  current  of  so  many  gallons  per  minute.*  A 
20-candle  Swan  lamp,  new  pattern,  takes  a current  of  about 
*7  ampere. 

Pressure. — V olt. 

The  unit  of  electric  pressure  is  called  the  Volt.  It  is 
analagous  to  steam-pressure  or  to  head  of  water.  We  speak 
of  an  electric  pressure  of  so  many  volts  as  we  might  speak 
of  a steam-pressure  of  so  many  pounds  to  the  square  inch, 
or  a head  of  water  of  so  many  feet.  One  DanielFs  cell  gives 

* In  the  case  of  the  water  flow  we  have  no  one  word  to  express 
the  strength  of  the  stream,  but  have  to  speak  of  quantity  per  time. 


1 1 


A mpere — Volt — Ohm . 

a pressure  of  nearly  one  volt.  One  20 -candle  Swan  lamp 
(new  pattern)  requires  a pressure  of  about  100  volts. 

Resistance. — Ohm. 

We  know  that  generally  a pipe  of  small  bore  offers  a 
greater  resistance  to  a flow  of  water  than  one  of  large  bore ; 
but  the  relation  between  the  resistances  of  different  pipes 
follows  extremely  complicated  laws.  We  know,  for  instance, 
that  a pipe  of  1 square  inch  bore  offers  more  resistance 
than  one  of  2 square  inches,  but  we  cannot  say  that  it  offers 
exactly,  or  even  approximately,  twice  the  resistance. 

Electric  resistance,  however,  as  we  have  already  stated,* 
follows  a very  simple  law.  For  a wire  of  a given  substance 
it  is  directly  proportional  to  the  length,  and  inversely  pro- 
portional to  the  cross  section.  Thus,  if  we  double  the 
length  of  a wire  we  double  its  resistance.  If  we  double  the 
section  we  halve  its  resistance.  If  we  double  the  diameter 
we  quadruple  the  section  and  reduce  the  resistance  to  one 
quarter.  If  we  double  the  length  and  double  the  section 
the  resistance  remains  unaltered. 

The  unit  of  electrical  resistance  is  called  the  Ohm.  It  is 
defined  as  the  resistance  at  a tempe  rature  of  0 C.,  of  a 
column  of  mercury  of  one  square  millimetre  section  and  of  a 
certain  length. 

The  value  f of  the  ohm  was  determined  in  1862,  by  a 
committee  of  the  British  Association;  and  the  result  of  their 
determination  is  that  the  standard  mercury  column  has  a 
length  of,  as  nearly  as  possible,  104  centimetres.  This  is 
the  value  which  is  at  present  in  practical  use.  It  may  be 
called  the  B.  A.  Ohm. 

Recent  investigations  have,  however,  shown  that  it  is 
about  1 per  cent,  too  low ; and  at  the  Congress  of  Electri- 
cians, which  met  in  Paris  on  Sept.  15, 1881,  an  International 
Commission  was  appointed  to  re-determine  it  with  all 
possible  accuracy.  Whatever  value  the  Commission  arrives 
at  is  to  be  called  the  “ Paris  Congress  Ohm,”  and  is  to  be 

# Page  6. 

t For  the  theory  ofthe  determination  of  the  Ohm,  see  my  “ Electricity,” 
2nd  edition,  vol.  i.  p.  303. 


12 


Electric  Lighting. 

adopted  permanently,  and  is  not  to  be  again  changed,  even 
if  a further  re- determination  should  show  that  it  is  not 
perfectly  accurate. 

To  get  some  idea  of  the  magnitude  of  the  ohm,  we  may 
note  that  a mile  of  No.  16  copper  bell-wire  has  a resistance 
of  about  14  ohms,  while  the  Atlantic  cable  has  a resistance 
of  about  7600  ohms.  The  carbon  thread  of  a 20-candle 
Swan  incandescent  lamp  (new  pattern)  has,  when  hot,  about 
143  ohms  resistance,  while  the  resistance  of  the  heated  air 
in  an  electric  arc  varies  from  6 ohms  to  1 ohm. 

Unit  op  Quantity. — Coulomb. 

The  unit  of  electrical  quantity  is  called  the  Coulomb,  and 
we  can  speak  of  a current  as  one  that  conveys  so  many 
Coulombs  per  second  through  the  wire.  A current  of  a 
strength  of  one  ampere  conveys  one  coulomb  per  second. 

Ohm’s  Law. 

The  three  units,  volt,  ohm,  ampere,  are  connected  by 
what  is  known  as  Ohm}s  law. 

Ohm’s  law  states  that  the  current  in  any  circuit  is  directly 
proportional  to  the  electro-motive  force,  and  inversely  pro- 
portional to  the  resistance,  and  the  units  are  so  chosen  that 
when  there  is  one  ohm  resistance  J in  circuit  an  electro- 
motive force  of  one  volt  produces  a current  of  one  ampere. 

We  see  then  that  two  volts  acting  through  one  ohm  would 
give  two  amperes,  or  one  volt  acting  through  two  ohms 
would  give  \ an  ampere. 

Ohm’s  law  may  thus  be  written, — 

„ , . v Electro-motive  force  in  volts. 

Current  m amperes  = — — — — J 

Kesistance  in  ohms. 

This  is  commonly  abbreviated  into  the  form, — 

c = | . . . . (1) 

This  may  also  be  written, — 

E = CE  • . (2) 


or,— 


(31 


Horse-power  and  Electric  Units.  13 

By  the  use  of  these  formulae  we  can  solve  problems  such  as 
the  following,  which  occur  daily  in  electric  lighting. 

(1.)  A machine  gives  an  electro-motive  force  of  60  volts. 
What  current  will  it  send  through  a resistance  of  5 ohms  ? 

We  have  from  (1), — 

n 60  1 f\ 

(J  = -V-  = 12  amperes. 

(2.)  What  electro-motive  force  must  a machine  have  to 
send  a current  of  2 amperes  through  a resistance  of  25  ohms? 

We  have  from  (2); — 

E = 2 x 25  = 50  volts. 

(3.)  What  is  the  resistance  of  a circuit  when  an  electro- 
motive force  of  800  volts  sends  a current  of  10  amperes 
through  it  ? 

We  have  from  (3); — 

R = = 80  olims. 

Units  of  Heat  and  Work,  and  their  relation  to  the 
Electrical  Units. 

Energy  and  Horse-Power. 

The  rate  at  which  Energy  is  being  expended,  as  for  in- 
stance in  maintaining  a current,  is  in  England  * commonly 
measured  in  “ horse-power.” 

One  horse-power  is  equal  to  550  foot-pounds  per  second, 
i.e.  can  raise  550  lbs.  1 ft.  per  second,  or  1 lb.  550  ft.  or 
10  lbs.  55  ft.  in  the  same  time. 

When  horse-power  is  being  expended  in  sending  a current 
through  a resistance,  the  conductor  offering  the  resistance 
is  heated.  The  quantity  of  heat  produced  per  minute  is 
equal  to  the  heat  which  must  be  expended  per  minute  in 
maintaining  the  current. 

The  horse-power  required  to  maintain  a current  is,  other 
things  being  equal,  proportional  to  the  square  of  the 
current.  Thus,  if  one  H.P.  could  maintain  one  ampere 


* In  France  it  is  measured  in  “ Force  des  Chevaux.” 
1 H.P.  = P0139  Force  de  Cheval. 


14  Electric  Lighting . 

through  a given  resistance,  4 H.P.  would  be  required  to 
maintain  2 amperes  through  the  same  resistance. 

The  heat  produced  in  the  conductor  is  proportional  to  the 
square  of  the  current.  Thus,  2 amperes  will  produce  four 
times  as  much  heat  in  a certain  wire  as  one  will. 

The  horse-power  required  to  maintain  a certain  current 
through  a resistance  is  proportional  to  the  resistance , and  he 
heat  produced  by  the  current  is  proportional  to  the  resis- 
tance. 

Corollary . The  heat  produced  per  unit  of  length  in  a 
wire,  on  which  depends  the  temperature  to  which  the  wire 
will  be  raised,  is  proportional  to  the  resistance  per  unit  of 
length. 

The  horse-power  required  to  maintain  a certain  current 
under  a certain  pressure , is  proportional  to  the  current 
multiplied  by  the  pressure. 

Calculations. 


Relation  between  Horse-Power,  Current,  and  Resistance. 

One  horse-power  can  maintain  a current  of  one  ampere 
through  746  ohms.  Or  one  of  two  amperes  through, — 

of  746  ohms,  &c. 

4 

This  is  expressed  generally  by  saying  that  the  horse- 
power required  to  maintain  a current  is  part  of  the 
square  of  the  current  in  amperes  multiplied  by  the  resistance 
in  ohms.  This  is  abbreviated  as  follows, — 


H.P.  = 


C2  R 
746 


This  may  also  be  written, — 


C2 


746  H.P. 
R 


(4) * 

(5) 


* 1 F.  de  C.  will  maintain  a current  of  1 ampere  through  736  ohms. 
Equation  (4)  becomes,— 

F.deC.=^ 


And  generally  H.P.  can  be  translated  into  F.  de  C.  by  substituting  736 
for  746  in  the  formulae. 


Horse-power , Current , and  E.M.F. 


15 


or,— 


it  = 


746  H.P. 

C2 


(6) 


Problem  (1). 

What  horse-power  is  required  to  maintain  a current  of  10 
amperes  through  a resistance  of  6 ohms  ? 

We  have  from  (4), — 


H.P.  = 


10  x 10  x 6 _ 600 
746  746 


or  a little  less  than  f of  a horse-power. 

(2.)  What  current  can  16  H.P.  maintain  through  a re- 
sistance of  64  ohms  ? 

We  have  from  (5), — 


C2  = 7 * — = I86  0. 

64 

whence  C = 13*65  amperes. 

(3.)  Through  what  resistance  can  10  H.P.  maintain  a 
current  of  2 amperes  ? 

We  have  from  (6), — 

R = 746  * = 1865  Ohms. 

2x2 


Belation  between  Horse-power,  Current,  and  E.M.F. 
Equation  (4)  shows  us  that,— 


H.P. 


C2R 

746 


(4) 


Equation  (3)  shows  us  that, — 


(3) 


Inserting  in  (4)  the  value  of  B given  by  (3)  we  have, — 


H.P.  = 


C2  E 
C 

746 


EC 

746 


• (7) 


or  the  horse-power  expended  in  sending  a current  through 
any  resistance,  constant  or  variable,  is  part  of  the  cur- 


i6 


Electric  Lighting . 


rent  in  amperes  multiplied  by  the  electro-motive  force  in 
volts  which  is  driving  it. 

Equation  (7)  may  also  be  written, — 


E = 


746  H P. 


or,— 


C = 


746  H.P. 
E 


• (8) 

• (9) 


Problem  (1). 

How  much  heat  will  be  developed  in  a circuit  by  a current 
of  18  amperes  driven  by  an  E.M.F.  of  200  volts  ? 

We  have  from  (7), — 

H.P.  = = 4*82  horse-power. 

(2.)  What  electro-motive  force  must  be  given  to  a machine 
in  order  that  5 H.P.  may  just  maintain  a current  of  25 
amperes  in  the  circuit. 

We  have  from  (8), — 


746  x 5 


149*2  volts. 


(3.)  A machine  has  an  E.M.F.  of  60  volts,  what  current 
will  be  developed  by  80  H.P.  ? 

We  have  from  (9), — 


C = 


746  x 80 
60 


= 994  amperes. 


Relation  between  Horse-power  Resistance  and  E.M.F. 


Equation  (7)  gives  us, 

EC 
746 


H.P; 


(7) 


Equation  (1)  gives  us, — 


(1) 


Substituting  in  (7)  the  value  of  C given  by  (1 ) we  have, — 
PE 

h li_  I? 

746  ~ 746  R 


H.P.  = 


(10) 


Work,  Quantity , and  E.M.F.  1 7 


This  may  also  be  written, — 
E3  = H.P.  746  R . 


or,— 


R = 


E2 

746  H.P.  • 


(11) 

(12) 


Problem  (1). 

What  H.P.  is  expended  by  an  E.M.F.  of  99  volts  working 
through  a resistance  of  140*5  ohms  ? 

We  have  from  (10), — 


H.P.  = 


99  x 99 
746  x 140-5 


•0936. 


(2.)  What  E.M.F.  will  be  developed  if  ^ of  a horse- 
power is  employed  in  sending  a current  through  30  ohms  ? 
We  have  from  (11), — 

Es  = -5-  746  x 30  = 2238. 


Whence  E = 47*3  volts. 


(3.)  What  should  be  the  resistance  of  a lamp  in  order 
that  when  placed  on  a machine  of  65  volts  E.M.F.  i of  a 
horse  power  may  be  expended  in  it  ? 

We  have  from  (12), — 

R = §5x65  = g3.9olims 
b 746 


Relation  between  Work,  Quantity,  and  E.M.F. 

If  we  have  a supply  of  water  under  constant  pressure, 
which  we  are  using  occasionally,  say  to  drive  a water  engine, 
we  can  tell  how  many  foot-pounds  of  energy  we  have  used 
at  the  end  of  a week,  if  we  know  the  pressure  and  the  total 
quantity  of  water  used. 

Similarly,  if  we  know  the  electro-motive  force  at  which 
our  electricity  is  supplied,  and  the  total  quantity  of  elec- 
tricity which  has  passed  through  our  circuits,  we  can 
calculate  the  total  quantity  of  energy  expended,  or  of  heat 
produced  in  the  resistance,  however  much  that  resistance 
may  have  been  varied  during  the  flow. 


c 


1 8 


Electric  Lighting. 


The  relation  is  given  by  the  equation, — 

W = *737  EQ  ....  (13) 

Equation  (13)  can  be  derived  from  equation  (7),  when  we 
remember  that  1 ampere  equals  1 coulomb  per  second, 
and  1 H.P.  = 550  foot-pounds  per  second.  The  number  of 
foot-pounds  expended  in  sending  a coulomb  through  the 
circuit,  is  therefore  550  times  the  number  of  H.P.  expended 
in  maintaining  an  ampere. 

(7)  thus  becomes, — 

W = |®EQ  = 737  E Q . . (13) 

Where  W is  the  work  expended  or  heat  generated  ex- 
pressed in  foot-pounds,  E is  the  electro-motive  force  in  volts, 
and  Q is  the  total  number  of  coulombs  of  electricity  which 
has  passed.* 

The  equation  may  also  be  written, — 

Q=T3Te  • • ■ • <14> 


y W 

h “ 737  Q 


(15) 


Problem  (1). 

With  a constant  E.M.F.  of  110  volts  how  much  work 
is  expended  in  sending  10,000  coulombs  through  a circuit 
of  varying  resistance  ? 

We  have  from  (13), — 

W = -737  x 110  x 10,000  = 810,700  foot-pounds. 


(2.)  How  much  electricity  will  33,000  foot-pounds  send 
through  a circuit  with  an  E.M.E.  of  60  volts? 

We  have  from  (14), — 

q = - 717^0  = 746  coulombs- 

(3.)  What  must  be  the  E.M.F.  in  a circuit  for  1474  foot- 
pounds to  send  10  coulombs  through  it  ? 

We  have  from  (15), — 

E = TsTVIo  = 200  TOlts' 


See  page  12. 


Relations  between  Mechanical  and  Electrical  U nits . 1 9 

Summary  op  Formulae. 

The  folldwing  is  a summary  of  the  various formula  which 
we  have  explained  : — 

C stands  for  Current  in  amperes. 

E „ E.M.F.  in  volts. 

R „ Resistance  in  ohms. 

Q „ Quantity  in  coulombs. 

H.P.  ,,  Rate  of  expenditure  of  work  in  horse-power. 

W „ Work  in  foot-pounds. 

1 H.P.  = 550  foot-pounds  per  se  cond  = 33,000  foot-pounds  per  minute. 


Horse-power. 


„pj’8 

HJo  746  

(4)  page  14. 

_E  C 

“ 746  

(7) 

99 

15. 

E2 

746  R 

(10) 

99 

16. 

Work. 

W = *737  E Q .... 

(13) 

99 

18. 

Current. 

C — - 

C“R 

(1) 

99 

12. 

/ 746  H.P.  * 

~\l  R ... 

(5) 

99 

14. 

746  H.P. 

E 

(9) 

>9 

16. 

Electro-motive  Force. 

E^CR 

(2) 

99 

12. 

746  H.P. 

- c 

(S) 

99 

16. 

= s/  H.P.  746  R . 

(11) 

99 

17. 

W 

•737  Q 

(15) 

99 

18. 

Resistance. 

E 



(3) 

99 

12. 

* The  symbol  \/  means  “ square  root  of’5  the  quantity  under  it. 

c 2 


20 


Electric  Lighting . 


746  H.P. 

(6)  page  15. 

~ C2  ' 

E 

“746  H.P. 

• (12)  „ 17. 

Quantity. 

W 

•737  E 

h-1 

M 

00 

The  Commeecial  Electeical  Unit. — Definition. 

The  unit  of  electrical  supply  is  defined  by  the  Board  of 
Trade  in  the  Provisional  Orders  to  be  1000  amperes  flow- 
ing for  one  hour  under  a pressure  of  one  volt. 

This  is  the  same  as  100  amperes  under  a pressure  of  10 
volts,  or  of  10  amperes  under  a pressure  of  100  volts,  or 
generally  as  1000  volt-amperes. 


Value  in  Hoese-Powee  pee  Houe. 

This  unit  is  mathematically  equal  to  1*34  actual  horse- 
power working  for  one  hour,  i.e.  just  over  1-^-  horse-power 
working  for  one  hour. 

For  we  have  by  the  formula  (7),  page  15, — 

E C 


H.P.  = 


746 


and  where 


E C = 1000 


H-p-  =im  = r34 


(16) 


Value  in  21 -Candle  Swan  Lamps  pee  Houe. 

A Swan  lamp  as  at  present  constructed  takes  exactly  ^th 
horse-power  when  working  at  21  candles.  Hence  the  com- 
mercial unit  is  a quantity  of  electricity  that  will  feed  13*4 
Swan  lamps,  each  of  21 -candle  power  for  one  hour. 


Value  in  14-candle  Swan  Lamps  pee  Houe. 

When  lamps  of  smaller  candle-power  are  used,  one  unit 
of  electricity  will  feed  a proportionably  larger  number  of 
them. 


The  Commercial  Unit. 


21 

One  commercial  unit  of  electricity  will  feed  \\  x 13*4 
equal  to  20  14- candle  Swan  lamps  for  one  hour. 

Equivalent  in  Gas. 

5 cubic  feet  of  gas  will  feed  one  burner  of  about  14  candles 
for  one  hour,  100  cubic  feet  of  gas  will  feed  20  14-candle 
burners  for  one  hour; 

Hence 

One  commercial  electrical  unit  (when  feeding  Swan  lamps) 
is  approximately  equal  in  illuminating  power  to  100  cubic 
feet  of  gas, 

Or, 

Ten  commercial  electrical  units  (when  feeding  Swan 
lamps)  are  approximately  equal  in  illuminating  power  to 
1000  cubic  feet  of  gas  . . . . . . (17) 

Rule  for  Comparison  of  Prices. 

We  see  from  the  above  that  to  compare  the  price  of 
electricity  with  that  of  gas  we  must  multiply  the  price  per 
electrical  unit  by  10,  and  the  result  will  be  the  price  of  a 
quantity  of  electricity  approximately  equal  in  illuminating 
power  to  1000  cubic  feet  of  gas. 


22 


Electric  Lighting . 


CHAPTER  IV. 

Rules  foe  the  Resistances  of  Divided  Circuits. 


Fig.  1. 


In  incandescent  electric  lighting  the  lamps  are  placed  so 
that  the  currents  leaving  the  mains  divide  between  them. 
A knowledge  of  the  right  way  to  calculate  the  resistance  of 
divided  circuits  is  essential  to  electrical  engineers. 

When  an  electric  circuit  consists  of  two  or  more  branches 

as  in  b,  c,  fig.  1,  the 

l current  divides  between 

— * 

them  in  the  inverse 
ratio  of  their  resist- 
ances. For  instance, 
if  the  branch  b has  twice  the  resistance  of  c,  then  -§-  of  the 
current  passes  through  c and  ^ through  b. 

When  wires  are  placed  side  by  side,  as  b}  c,  fig.  1,  so 
that  the  current  divides  between  them,  they  are  said  to  be 
“ in  parallel  circuit,”  or  “in  multiple  arc,”  or  “in  quantity;” 
all  three  expressions  are  used. 

When  they  are  ar- 

a'  ^ « c tr  ds  ranged  end  to  end,  as 

*^•2*  in  fig.  2,  they  are  said 


to  be  “ in  series.” 

We  see  that  in  the  “Quantity”  arrangement  the  total 
resistance  of  the  divided  circuit  is  less  than  that  of  either  of 
its  branches,  for  the  two  wires  side  by  side  are  equivalent 
to  one  whose  cross-section  is  equal  to  the  sum  of  the  cross- 
sections  of  the  branches. 

If  the  resistances  of  all  the  branches  are  equal , the  total 


23 


Quantity  and  Series. 

resistance  of  the  whole  circuit  is  the  resistance  of  one  branch 
divided  by  the  number  of  branches  ....  (18) 

Problem : — 

If  we  have  20  incandescent  lamps  connected  in  quantity 
(fig.  3),  and  each  has  a resistance  of  125  ohms,  what  is  their 
total  resistance  ? 

We  have  from  (18), — 


Fife.  3. 


If  the  lamps  are  connected  in  series,  their  total  re- 
sistance is  the  resistance  of  one  multiplied  by  the  number  in 
series  . . . . . . . . . (19) 

Problem  : — 

If  we  have  3 of  the  same  lamps  connected  in  series  (fig.  4) 
what  is  their  total  resistance  ? 

We  have  from  (19),— 

R,  = 125  x 3 = 375  ohms.  — to -vu eo 

Fig.  4?. 

We  sometimes  have  to  arrange  a number  of  equal  re- 
sistances, such  as  incandescent  lamps,  partly  in  quantity, 
partly  in  series,  i.e.  each  branch  of  our  parallel  circuit  is 
made  up  of  two  or  more  resistances  in  series. 

The  total  resistance  P of  the  circuit  is  then  given  by  the 
following  formula,  where  r is  the  resistance  of  one  lamp,  s 
the  number  of  lamps  in  series  in  each  branch,  and  q the 
number  of  branches  arranged  in  quantity. 

R = y (20) 

Problem  : — 

What  is  the  resistance  of  20  lamps  of  25  ohms  each, 
arranged  in  5 branches,  each  consisting  of  4 lamps  in  series 
(fig.  5)  ? 

We  have, — 


r = 25  ohms,  s = 4.  q = 5. 


24  Electric  Lighting . 

And  we  have  from  (20), — 


k 

3 

S 

* f 

J8 

1 

f 

£ 

§ 

s 

3 

I 3 

\ 

? 

i 

5 f 

E = 


25  x 4 


20  ohms. 


Pit-  5. 

When  the  different  branches  have  not  all  the  same  re- 
sistance, the  formula  for  determining  the  total  resistance  is 
more  complicated. 

Let  rlf  r2,  r3,  &c.  (fig.  6),  be  the  whole  resistance  of  each 
of  the  branches  respectively;  each  may  be  one  resistance, 
or  made  up  of  several  smaller  ones  in  series. 

The  total  resistance 

9 R of  the  circuit  is 

given  by  the  formula. 


E = 


r2  r3  &c. 


r2  r3  &c.  + A r3  &c.  + r2  &c.  + &c. 


. (21)* 


Problem  : — 

What  is  the  resistance  of  a divided  circuit  of  5 branches, 
the  total  resistance  of  each  branch  respectively  being  4,  12, 
5,  100,  and  3 ohms  ? 

We  have  from  (21), — 


E = 


(4  x 12  x 5 x 100  x 3) 


(12  x o x 100  x 3)  + (4  x 5 x 100  x 3)  + (4  x 12  x 100  x 3) 
+ (4  x 12  x 5 x 3)  + (4  x 12  x 5 x 100) 


74, 4°°  _1.1,  , 

~ 18,600  + 6200  + 14,880  + 744  + 24,800  “ 

It  is  often  required  to  construct  a resistance,  such  that  a 
known  fraction  of  the  whole  current  shall  go  through  one 
branch. 

For  instance,  if  we  wish  to  measure  a strong  current  by 


* In  this  formula  the  numerator  is  the  product  of  all  the  resistances, 
while  each  term  of  the  denominator  is  the  product  of  all  except  one,  each 
one  being  omitted  in  turn.  We  also  note  that  each  term  of  the  denomi- 
nator consists  of  the  numerator  divided  by  the  resistance  which  has  been 
omitted  in  that  term.  This  saves  labour  in  the  calculation. 


Galvanometer  Shunts . 


25 


a “ galvanometer, 99  * only  constructed  for  the  measurement 
of  feeble  currents, 


we  may  arrange  it 
with  a “ shunt 99  as 


1 0 


Fig.  7. 


it  is  called,  as  in 
fig.  7,  so  that  say 

of  the  current  goes  through  the  galvanometer  e and  ^ 
through  the  shunt  b.  The  galvanometer  gives  us  the  value 
of  yjj-  of  the  current,  and  10  times  this  is  the  whole  value  of 
the  current. 

The  general  rule  for  the  construction  of  shunts  is  the 
following : — 

If  it  is  desired  to  send  i of  the  current  through  any  in- 
strument, and  the  rest  of  it  through  the  shunt,  the  resistance 
of  the  shunt  must  be, — 

°f  the  resistance  of  the  instrument  J . . (22) 

Problem : — 

We  wish  to  measure  a strong  current  by  means  of  a 
galvanometer  of  880  ohms  resistance,  and  to  send  exactly 
part  of  the  current  through  the  galvanometer.  What 
must  be  the  resistance  of  the  shunt  which  is  to  be  placed  in 
parallel  circuit  with  the  galvanometer  ? 

We  have  from  (22), — 

Resistance  of  shunt  = — - resistance  of  galvanometer 


99 


X 880  = 8’88  ohms. 


(2.)  What  must  be  the  resistance  of  the  shunt  for  J of  the 
current  to  go  through  a lamp  of  120  ohms  ? 

1 


Resistance  of  shunt  = 


4-1 


120  = 40  ohms. 


* See  below,  Chapter  Y. 
f The  symbol 


indicates  a battery,  and  is  used  with  that  meaning  throughout  this  book. 

+ For  the  theory  of  this  rule  see  my  “ Electricity,”  2nd  edition,  vol.  i- 
p.  276. 


26 


Electric  Lighting . 


Self-adjustment  of  Current  by  Lamps  in  Quantity. 

Suppose  we  have  a generator  of  very  small  internal  resis- 
tance and  of  constant  E.M.F.,  and  we  supply  a current  from 
it  to  a number  of  lamps  in  quantity,  the  current  in  each  lamp 
will  be  sensibly  the  same,  whether  the  number  of  lamps 
connected  is  (within  certain  limits)  great  or  small,  i.e.  if  we 
have  a number  of  lamps  connected  we  can  extinguish  as 
many  of  them  as  we  please  without  sensibly  affecting  the 
remainder. 

For  let  E be  the  E.M.F.  of  the  generator,  g its  internal 
resistance,  r the  resistance  of  one  lamp,  n the  number 
of  lamps.  Thus,  from  (18)  the  total  resistance  of  the  are 


The  current  being  divided  between  n lamps,  the  current  c 


We  see  that  when  g is  very  small,  this  is  nearly  inde- 
pendent of  the  value  of  n , i.e.  is  nearly  the  same  whether 
many  or  few  lamps  are  in  the  circuit. 

This  will  be  the  case  when  g is  the  internal  resistance  of 
a battery  with  very  large  plates.  With  dynamo  machines, 
however,  the  apparent  resistance  is  so  much  increased  by 
“ self-induction  ”■  * that  the  self-adjustment  only  takes  place 
over  a very  limited  range,  and  other  means  of  keeping  the 
electro-motive  force  constant  have  to  be  used,  which  will  be 
discussed  in  due  course. 

Method  of  calculating  the  H.P.  wasted  in  a Network  of 


T 

lamps  will  be  - 

n 

and  from  (1)  the  total  current  will  be 

TT.  v,  TT. 


. (23) 


n r + ng  r + ng 


Conductors  supplying  Lamps. 

In  all  systems  of  electric  lighting  it  is  important  to  know 
what  proportion  of  the  electricity  generated  is  utilized  in 


* See  Chapter  IX. 


H.P.  wasted  in  Conductors . 


27 


the  lamps,  and  what  proportion  is  wasted  in  heating  the 
conductors. 

When  we  have  a rule  for  determining  this  we  can  pro- 
perly apportion  the  diameter  of  each  conductor  to  the  current 
it  has  to  carry,  and  to  the  distance  to  which  it  has  to  carry 
it ; so  that,  on  the  one  hand,  we  may  not,  by  making  the 
conductor  too  small,  expend  too  great  a quantity  of  coal  in 
forcing  the  current  through  it ; or,  on  the  other  hand,  by 
making  it  too  large,  so  increase  our  capital  expenditure  on 
copper  that  the  interest  on  it  is  too  large  a proportion  of 
the  annual  rental  which  we  can  charge  for  the  electricity 
used  in  the  lamps. 

When  the  same  current  passes  through  two  resistances, 
such,  for  instance,  as  a wire  and  the  lamps  fed  by 
it,  the  horse-powers  expended  in  the  two  resistances 
respectively,  are  simply  proportional  to  the  resistances.  For 
by  the  formula  (4),  p.  14,  if  r and  r are  the  two  resist- 
ances, the  horse-powers  expended  by  the  same  current  C are 

HP  — °2  r 

H-R  “ 746 

and 

CV 


H.P/  = 


and  their  ratios  are 


746’ 


C2  r 

H.P.  _ 746  _ r . . . . (24i 

H.P/  C2  / r' 

746 

When,  as  in  arc-lighting,  the  lamps  are  all  placed  in 
series,  the  determination  of  the  relative  horse-powers  is  very 
simple,  for  the  wire  is  of  uniform  section  throughout,  and  its 
total  resistance  is  its  resistance  per  yard  multiplied  by  its 
length  in  yards. 

The  resistance  of  each  lamp  is  known,  and  the  total  lamp- 
resistance  is  the  sum  of  these  resistances. 

Example.  16  arc  lamps,  each  of  2T  ohms  resistance, 
are  placed  on  a circuit  450  yards  long,  consisting  of  a wire 
having  a resistance  of  *006  ohms  per  yard.  What  proportion 
of  the  horse-power  is  used  and  wasted  respectively  ? 

The  lamp-resistance  r — 16  x 2T  = 33’6  ohms. 


28 


Electric  Lighting. 


The  wire-resistance  r = 450  x *006  = 2*7  ohms. 

The  ratio 

H.P.'  _ / _ 2-7  _ 

H.P.  r 336 

or  8 per  cent. 

Note. — We  must  be  careful  not  to  confuse  the  ratio  of 
horse-power  wasted  to  horse-power  used,  with  the  ratio 
of  horse-power  wasted  to  total  horse-power. 

The  latter  is  the  ratio  of  wasted  horse-power  to  the  sum 
of  the  wasted  and  used  horse-powers ; or, 

ll.R  + l'l.lV <25> 


This,  in  the  case  when  the  current  is  the  same  throughout 
the  circuit,  still  depends  only  on  the  resistances,  and  is  given 
by  the  formula 


H.P/  _ * 

H.r.  -j-  n x!  r -f  r' 


(26) 


With  a circuit  as  given  in  the  previous  example,  the  ratio 
of  the  horse-power  wasted  to  the  total  horse-power  would  be 

33-62+  2-7  = '07i’ m 7'4  per  Cent 
With  one  group  of  incandescent  lamps,  either  in  quantity 
series,  or  of  any  combination  of  the  two  placed  at  the  end  of 
a pair  of  leads,  as  in  fig.  8,  the  problem  of  the  deter- 
mination of  the  relative  horse-powers  wasted  and  used,  is 


equally  simple ; for  the  wire  being  of  uniform  section,  we 
know  its  resistance,  and  the  resistance  of  the  group  of  lamps 
is  given  by  the  formula  (20)  of  page  (23). 

The  current  in  every  part  of  the  leads  being  the  same 
as  the  current  in  the  group  of  lamps,  the  relative  horse- 
powers are  still  proportional  to  the  relative  resistances. 

We  see  in  all  these  'problems  that  the  longer  the  conductor 
is,  the  thicker  it  must  he,  for  if  a given  conductor  wastes  a 
certain  horse-power , and  we  wish  to  double  its  length,  i.e. 
to  put  the  lamps  twice  as  far  from  the  machine,  without 


H.P.  wasted  in  a System  of  Street-mains.  29 

increasing  the  waste,  we  must  also  double  its  sectional  area, 
so  as  to  keep  its  resistance  cojistant,  that  is,  we  must  quadruple 
its  weight  * 

In  practical  incandescent  lighting,  however,  the  lamps 
are  distributed  at  intervals  along  the  pair  of  conductors  as 
in  fig.  9,  and  the  problem  at  once  becomes  much  more 


Fig.  9. 


complex,  because  different  parts  of  the  conductors  are 
carrying  currents  of  different  strengths,  and  the  simple 
formula  (25),  page  28,  is  no  longer  applicable. 

For  we  have  in  fig.  9 

The  portion  a of  the  conductor  carries  the  current  of  1 lamp. 

„ b „ „ 2 lamps. 

» c » „ 3 „ 

>}  d ,,  ,,  4 ,, 

If  we  consider  the  conductors  + d a and  — da  in  fig.  9 
to  be  the  wires  laid  along  a side  street,  then  the  branches 

L,2,  &c.,  will  not  be  single  lamps,  but  may  each  be  con- 
sidered to  represent  the  whole  group  of  lamps  in  one  house; 
while,  if  we  consider  the  conductors  to  be  the  mains  in  a prin- 
cipal street,  the  branches  Jjy  L2,  &c.,  may  be  considered  as  re- 
presenting the  sub-mains  branching  into  the  side  streets. 

We  see  that  Lx  L2,  &c.,  are  not  necessarily  equal  to  one 
another. 

In  order  to  determine  the  relative  horse-powers  used  and 
wasted  in  a system  of  town  supply,  1 prefer  to  use  a method 
which  I communicated  to  the  Society  of  Telegraph- 
Engineers  on  Dec.  13,  1883. 

We  first  mark  out  on  a large-scale  map  of  the  district  the 
number  of  lamps  likely  to  be  required  in  each  block  or 
house. 

We  then  draw  the  street-mains  and  branches  radiating 

O 

from  the  engine-house  to  the  houses  to  be  lighted. 

We  then,  starting  from  the  farthest  points  on  each  branch, 
* See  page  11. 


30 


Electric  Lighting . 

work  up  towards  the  engine-house,  marking  on  each  branch 
and  main  the  number  of  lamps  it  has  to  carry. 

Plate  I.  is  an  example  of  a district  so  marked  out;  the 
plain  numbers  being  the  number  of  lamps  * in  the  block  or 
house  on  which  they  are  marked,  and  the  numbers  surrounded 
by  a circle  being  the  number  of  lamps  carried  by  the  wire  near 
which  they  are  written. 

To  avoid  confusion,  the  + conductor  only  is  shown,  and 
when  the  H.P.  wasted  in  it  has  been  obtained,  the  result  must 
be  doubled  to  obtain  the  total  waste  in  the  + and  — con- 
ductors. Knowing  the  current  used  per  lamp,  we  know  the 
number  of  amperes  which  each  wire  has  to  carry. 

We  note  that  the  “ carrying”  number  in  each  branch  is 
the  sum  of  all  the  numbers  beyond  it,  i.e.  on  the  side 
furthest  from  the  dynamo  in  that  branch. 

In  order  to  secure  the  greatest  economy  of  copper  and  of 
coals,  the  section  of  the  conductor  must  be  directly  proportional 
to  the  current  it  has  to  carry,  i.e.  as  we  leave  the  dynamo,  the 
section  of  the  conductor  must  diminish  after  each  branch  leaves 
it , in  order  that  the  same  number  of  amperes  per  square  inch 
may  be  carried  by  every  part  of  the  conductor  throughout  the 
system,  f 

This  condition  being  given,  then,  for  a given  district 
mapped  out,  the  percentage  of  H.P.  wasted  in  the  conductors 
is  simply  proportional  to  the  number  of  amperes  per  square 
inch  which  we  use. 

By  the  method  we  are  about  to  explain,  it  is  easy  to 
calculate  the  percentage  of  horse-power  wasted  for  any  given 
number  of  amperes  per  square  inch. 

In  order  to  find  the  amperes  per  square  inch  corresponding 
to  the  particular  percentage  horse-power  that  we  are  prepared 
to  waste,  we  must  assume  some  number  of  amperes  arbi- 

* The  number  of  lamps  need  not  be  quite  the  total  number  erected, 
but  should  be  the  total  number  likely  to  be  ordinarily  in  use.  It  is  not 
necessary  to  provide  copper  to  be  always  in  position  for  lamps  that  are  only 
lighted  occasionally.  In  putting  on  extra  lamps  for  short  periods,  care 
must  however  be  taken  that  the  heating  limit  is  not  approached.  This 
will  be  discussed  in  the  chapter  on  “ Fire-risks.’’ 

t For  the  mathematical  proof  of  this,  see  Appendix. 


H.P . wasted  in  a System  of  Street-mains . 3 1 

trarily,  and  find  tlie  actual  horse-power  wasted,  and  then  the 
required  number  of  amperes  will  bear  the  same  ratio  to 
the  required  horse-power  that  the  arbitrarily-assumed  num- 
ber does  to  the  horse-power  corresponding  to  it  . . (27) 

For  example : — 

Suppose  in  a system  where,  say  100  H.P.  is  being  used 
in  lamps,  we  are  prepared  to  waste  12|  H.P.,  and  that  with 
an  assumed  current  of  500  amperes  per  square  inch  we  find 
(by  the  method  of  calculation  which  we  are  about  to  give) 
that  we  waste  16  H.P.,  then  the  right  number  of  amperes 
per  square  inch  for  us  to  use  is 

12i 

— - x 500  = 390  amperes  per  square  inch. 

And  if  we  have  calculated  the  section  of  copper  on  the 
basis  of  500  amperes  per  square  inch,  we  must  increase 
the  section  in  the  ratio  of  16  to  12^. 

Calculation  of  H.P.  wasted. 

We  now  come  to  the  method  of  calculating  the  horse- 
power wasted  in  a system  of  conductors,  when  a current 
of  a certain  number  of  amperes  per  square  inch  is  flowing 
through  it. 

When  there  are  the  same  number  of  amperes  per  square 
inch  in  a system,  the  horse-power  wasted  in  each  cubic  inch  of 
copper  is  the  same  throughout  the  whole  system  or  district. 

The  resistance  between  the  two  faces  of  an  inch  cube 
of  copper  is  *0000007  (seven  ten-millionths)  of  an  ohm. 

The  horse-power  (which  we  will  call  H.P.)  expended  in 
a cubic  inch  of  copper  with  a current  of  C amperes  per 
cubic  inch,  is 

SE=~CiX^-g (28) 

746 

With  500  amperes  per  square  inch,  the  horse-power  per 
cubic  inch  is 

HP  ~ 5002  x *0000007  _ .000234  . . (29) 

746 

When  we  know  this  constant,  H.P.,  and  also  the  total 
number  of  cubic  inches  of  copper  in  the  district,  we  know 
the  total  horse-power  wasted  in  the  district. 


32 


Electric  Lighting. 


To  determine  the  number  of  cubic  inches  of  copper  in 
the  district,  we  return  to  our  map,  on  which  the  number  of 
lamps  on  each  branch  is  marked,  and  we  calculate  a constant 
for  the  area  of  copper  per  lamp  corresponding  to  our 
assumed  current  of  say  500  amperes  per  square  inch. 

For  instance,  if  each  lamp  takes  *85  ampere,  then  for 
each  lamp  we  must  have 


^85 

500 


= *0017  = square  inch  of  copper. 


We  then  multiply  the  number  of  lamps  on  each  section 
of  the  branch  or  main  by  this  new  constant,  and  we  get 
the  required  area  of  this  section,  which  we  can  mark  upon 
it  on  the  map. 

We  next  multiply  the  area  of  each  section  by  its  length 
in  inches,  and  this  gives  us  its  volume  in  cubic  inches. 

Adding  all  the  results  thus  obtained  together,  we  get  the 
total  number  of  cubic  inches  of  copper  in  the  positive  leads 
throughout  the  district. 

Twice  this  result  is  the  total  amount  in  the  + and  — 
leads  together. 

Multiplying  this  result  by  H.P.,  the  horse-power  constant 
(which,  when  C = 500,  is  equal  to  *000234),  we  get  the  total 
horse-power  wasted  in  the  copper  in  the  whole  system. 

If  the  H.P.  wasted  is  more  or  less  than  the  desired  amount, 
we,  as  we  said  before,  alter  C proportionately  to  the 
desired  change  in  the  H.P. 

We  of  course  know  the  H.P.  expended  in  the  lamps,  as 
we  know  the  number  of  lamps  and  the  H.P.  expended  in  each. 

If  H.P.l  is  the  total  H.P.  used  in  the  lamps,  and  H.P.W 
the  total  H.P.  wasted,  then  the  percentage  Pw  of  the  whole 
H.P.  expended  which  is  wasted  in  the  leads,  is 

_ 100  H.P.w 

w “ H.P.l  + H.P.w* 


As  far  as  we  have  yet  gone,  we  have  assumed  that  all 
the  lamps  are  alight  whenever  the  current  is  flowing. 

In  practice  this  will  not  be  the  case,  and  we  must  note 
that  if,  the  conductors  remaining  unchanged,  we  diminish 


Total  Energy  wasted  in  a Sy stein  of  Conductors.  33 

the  number  on  every  branch  in  a certain  uniform  ratio,  we 
shall  diminish  the  wasted  H.P.  in  the  square  of  that  ratio. 

That  is,  if,  when  1000  lamps  are  burning,  we  are  using  100 
H.P.  and  wasting  10  H.P.,  then,  if  we  reduce  the  number  of 
lamps  to  500,  we  shall  reduce  the  used  H.P.  in  the  ratio  of 
■^3%-,  i.e.  to  50  H.P.,  but  we  shall  reduce  the  wasted  H.P. 
in  the  ratio  of  (^Q°Q0Q)2,  or  to  J of  its  former  amount,  namely, 
to  24  H.P. 

To  put  this  in  symbolical  form,  we  may  say  that,  with  a 
given  system  of  conductors,  if 

H.P.wm  is  the  H.P.  wasted  when  M lamps  are  burning, 

and 

H.P.wn  ,,  „ N ,,  ,, 

then 

H-P-WN=  (!)’  H.P.wm (30) 

This  formula  is,  as  we  said  above,  only  correct  when  the 
number  of  lamps  diminishes  uniformly  over  the  whole 
system,  i.e.  when  an  equal  proportion  of  the  lamps  in  every 
block  are  turned  out  simultaneously. 

In  districts  containing  the  same  class  of  houses,  the  con- 
dition is  sufficiently  nearly  approximated  to  in  practice  to 
make  the  formula  (30)  a useful  one  in  calculating  probable 
waste.  Assuming,  then,  this  condition,  we  can,  if  we 
know  the  general  average  habits  of  the  district  as  to  the 
use  of  light,  calculate  the  total  relative  quantities  of  coals 
which  will  be  used  in  the  engines  in  producing  useful  and 
wasted  electricity  respectively. 

We  will  use  the  symbol  H.P.H.  for  “ horse-power-hour/ 
i.e.  for  a H.P.  working  for  an  hour.  Thus,  20  H.P.  working 
for  3 hours  would  be  equal  to  60  H.P.H. 

The  coals  used  in  an  engine  are  practically  proportional 
to  the  H.P.H.,  i.e.  to  the  H.P.  developed,  multiplied  by  the 
hours  during  which  the  engine  works.  In  order  to  determine 
the  ratio  of  the  coals  used  in  producing  wasted  and  useful 
electricity,  we  must  take  the  H.P.H.  used  and  wasted  hour 
by  hour  throughout  the  night. 

This  will  be  best  understood  by  an  example. 


D 


34 


Elect7ric  Lighting . 

Suppose  that  we  have  1000  lamps  and  such  a system  of 
mains,  that,  when  all  the  lamps  are  on,  we  use  100  H.P. 
and  waste  10,  and  suppose  that  the  number  of  lamps  in  use 
at  the  different  parts  of  the  night  are  as  in  the  first  two 
columns  of  the  following  table,  then  the  H.P.H.s  used  and 
wasted  will  be  as  in  the  fifth  and  sixth  columns  respectively, 
where  the  letters  L and  W stand  for  “ used  in  lamps  39  and 
“ wasted  33  respectively. 


Hours. 
| P.M. 

Lamps 

burning. 

h.p.l 

H.P.W 

1 h.p.h.l 

1 

H.P.H.W 

j 

Before  5 

Very  few 

Inappreciable 

5-6 

100 

10 

T 

10 

•1 

6—7 

500 

50 

2*5 

50 

2.5 

7—10 

1000 

100 

10*0 

300 

300 

10—11 

800 

80 

6-4 

80 

6*4 

11—12 

400 

40 

1-6 

40 

1-6 

12 — 2 a.m. 

200 

20 

•4 

40 

■8 

j After  2 

Very  few 

Inappreciable 

Total 

520 

41*4 

Thus,  although  the  percentage  H.P.  wasted  when  all  the 
lamps  are  on  is 

IV  = 100  Jqq1^  = 9*9  per  cent., 

yet  the  percentage  of  coals  wasted  in  the  whole  night  is 
only 

41-4 

520  + 41-4 


Pw  = 100 


= 7*3  per  cent. 


-THE  SIEMENS  ELECTRO-DYNAMOMETER. 


35 


CHAPTER  V. 

EXPERIMENTAL  MEASUREMENT  OP  CURRENT — ELECTRO -MOTIVE 
PORCE — RESISTANCE — AND  HORSE-POWER  DEVELOPED  IN  RESIS- 
TANCE. 


Measurement  op  Current-  by  the  Siemens  Electro-dyna- 
mometer. (Plate  II.) 

The  principle  on  which  the  electro-dynamometer  is  founded 
is  the  fact  that  two  neighbouring  wires  carrying  currents 
attract  each  other  if  the  currents  are  in  the  same  direction, 
and  repel  if  they  are  in  opposite  directions. 

The  instrument  as  constructed  by  Messrs.  Siemens  con- 
sists of  a fixed  coil  of  wire  (Plate  II.)  of  the  shape  of  a flattened 
ring,  and  a ring  of  one  or  more  turns  of  stout  wire  sus- 
pended by  a spiral  spring.  The  plane  of  the  suspended  ring 
in  its  position  of  rest  is  at  right  angles  to  that  of  the  fixed 
ring.  The  two  ends  of  the  suspended  ring  dip  into  mer- 
cury cups,  which  allow  a current  to  be  sent  round  it  while 
it  is  still  quite  free  to  turn.  The  wires  are  connected  so 
that  a current  entering  the  instrument  passes  through 
both  the  fixed  and  suspended  coils. 

The  ring  suspended  by  the  spiral  spring  has  its  upper 
end  attached  to  a nut  or  button  called  a “ torsion  head.-” 
The  latter  carries  a pointer,  which,  when  the  torsion  head 
is  turned  by  hand,  moves  over  a scale  of  degrees,  and  indi- 
cates through  what  angle  the  top  end  of  the  spring  has 
been  twisted. 

When  a current  is  sent  through  the  instrument  the  sus- 

D 2 


36 


Electric  Lighting. 

pended  coil  is  deflected,  but  is  prevented  moving  more  than 
about  5°  by  a stop.  The  torsion  head  is  then  turned  by 
hand  until  the  twist  or  torsion  of  the  spring,  acting  against 
the  current,  brings  the  suspended  ring  back  to  its  zero 
position.  The  number  of  degrees  through  which  the  torsion 
head  has  had  to  be  turned  is  a measure  of  the  strength  of 
the  current.  A table  is  supplied  with  each  instrument,  show- 
ing the  number  of  amperes  corresponding  to  each  degree  of 
twist.  The  table  is  prepared  by  comparing  the  indications 
of  each  instrument  with  those  of  an  absolute  electro  dyna- 
mometer,* when  the  same  currents  are  sent  through  both 
instruments.  Some  of  the  instruments  have  two  fixed  coils, 
one  consisting  of  a good  many  turns  for  feeble  currents,  the 
other  of  a few  turns  for  strong  currents.  Such  instruments 
of  course  have  two  reduction  tables. 

Thus  to  measure  a current  with  this  instrument,  we  first 
level  the  instrument  carefully,  and  adjust  it  so  that  the  sus- 
pended coil  hangs  at  its  zero  position.  If  the  instrument  is 
in  proper  order,  this  will  be  when  the  torsion  pointer  is  also 
at  zero.  W e then  send  the  current  through  it,  and  then 
turn  the  torsion  head  until  the  suspended  coil  returns  to 
zero.  We  then  look  in  the  table  to  see  what  current  corre- 
sponds to  the  reading  of  the  torsion  pointer. 

It  sometimes  happens  that  owing  to  the  instrument  being 
a little  out  of  order,  the  torsion  head  has  to  be  turned  a few 
degrees  from  zero,  in  order  to  bring  the  suspended  coil  to 
its  zero  when  no  current  is  passing.  When  this  has  to  be 
done,  the  zero  error  must  be  subtracted  from  or  added 
to  the  reading  of  the  torsion  needle,  to  give  the  amount 
of  torsion  balancing  the  current. 

For  instance,  suppose  when  no  current  is  passing,  that  in 
order  to  bring  the  coil  to  zero,  the  torsion  needle  has  to  be 
moved  4°  in  the  same  direction  as  that  in  which  it  is  after- 
wards to  be  moved  to  balance  the  current ; and  that  its 
position  when  the  current  is  balanced  is  at  the  20°  mark. 

Then,  in  order  to  balance  the  current,  we  have  moved  the 
torsion  needle  from  4°  tp  20°,  that  is  through  16°,  and  our 


* See  my  ‘‘Electricity.”  2nd  Edit.  vol.  ii.  p.  79. 


Galvanometers . 


37 

current  will  be  that  corresponding  not  to  20°,  but  to  16°  in 
the  table. 

If  we  bad  previously  bad  to  move  tbe  torsion  bead  4°  in 
tbe  opposite  direction  and  it  balanced  tbe  current  at  20°,  we 
should  bave  bad  to  move  it  from  — 4°  to  20°,  i.e.  through 
24°;  and  our  current  will  be  that  corresponding  to  24°  in 
tbe  table. 

Tbe  chief  advantage  of  tbe  instrument  is  that  it  measures 
cc  alternating  ” currents  as  well  as  direct  ones,  for  tbe  at- 
traction simply  depends  on  tbe  currents  in  tbe  coils  being  in  tbe 
same  direction,  and  is  not  affected  if  they  are  both  reversed. 
This  is  important,  as  a large  class  of  tbe  machines  used  in 
electric  lighting  give  currents  whose  direction  is  reversed 
many  times  a second.  It  cannot,  however,  be  regarded  as 
an  extremely  accurate  instrument,  and  is  open  to  tbe  great 
objection  that  each  measurement  takes  some  time,  and  that 
therefore  it  will  give  no  information  as  to  sudden  or  momen- 
tary variations  of  the  current,  such  as  take  place  when  a 
lamp  is  out  of  order  and  tbe  light  is  flickering. 

GrALVANOMETEBS. 


Fig.  10. 


If  a wire  be  placed  parallel  to  a magnetic  needle,  as  in 
fig.  10,  a current  passing  along  tbe  wire  tends  to  set 
tbe  magnetic  needle  at  right 

angles  to  it.  If  tbe  motion  ~ | ^ 

be  opposed  by  some  force  such 
as  tbe  eartb^s  magnetism,  tbe 
effect  of  which  on  the  needle 
gets  greater  as  tbe  deflection  increases,  tbe  amount  of 
deflection  will  depend  on  tbe  strength  of  tbe  current ; and  if 
tbe  opposing  force  does  not  change,  tbe  same  current  will 

always  produce  tbe  same  deflec- 

tion.  If  the  wire,  instead  of  f _ 
passing  once  over  tbe  needle,  as  ( 

in  fig.  10,  passes,  say  four  times  ^ 

round  it,  as  in  fig.  11,  the  effect  ^ 

on  tbe  needle  will  be  four  times  Fig.  n. 

as  great  for  tbe  same  current. 

By  properly  proportioning  tbe  number  of  turns  and  the 


38 


Electric  Lighting. 

force  tending  to  bring  the  needle  back  to  zero,  we  can  con- 
struct a Galvanometer,  as  it  is  called,  which  will  conveniently 
measure  currents  of  any  strength. 

The  Reflecting  Galvanometer. 

For  the  measurement  of  very  feeble  currents,  Sir.  Wm. 
Thomson's  Reflecting  Galvanometer  (Plate  III.)  is  used. 

Its  construction  is  as  follows  : — 

Two  coils  of  wire  are  used,  round  which  the  current  goes 
in  opposite  directions.  Magnets  rigidly  connected  to  each 
other  are  suspended  in  each.  The  similar  poles  of  the 
magnets  are  turned  in  opposite  directions.  The  directive 
action  of  the  earth  or  of  the  setting  magnet  is  thus  very 
feeble,  as  it  is  only  equal  to  the  difference  of  the  actions  on 
the  two  magnets.  The  actions  of  the  coils  are  added 
together. 

The  instrument  is  thus  very  sensitive. 

The  Lamp,  Scale,  and  Mirror. 

To  detect  and  measure  small  angular  deflections  of  a 
needle,  a long  pointer  is  necessary ; but,  if  a long  material 
pointer  were  attached  to  the  needle,  its  weight  would  destroy 
the  sensitiveness  of  the  instrument. 

Sir  Wm.  Thomson  has  therefore  arranged  a method  by 
which  a beam  of  light  is  made  to  act  as  a pointer  of  any 
length,  and  absolutely  without  weight. 

A circular  mirror,  about  J of  an  inch  in  diameter,  is  rigidly 

A lamp  and  scale,  of  which  the 
back  (that  is,  the  side  furthest 
from  the  galvanometer)  is  shown 
in  fig.  12,  is  placed  on  the  table 
about  two  feet  from  the  instru- 
ment. The  light  passes  through 
a small  opening  in  the  lower  part 
of  the  scale,  fails  on  the  mirror, 
and  is  reflected  on  to  the  upper 
part,  making  a spot  of  light.  The 
least  motion  of  the  needle  and 
mirror,  of  course,  moves  the  spot  along  the  scale.  The 
distance  which  it  moves  is  equal  to  that  which  would  have 


attached  to  the  needle. 


Fig.  12. 


Plate  III, — Thomson’s  reflecting  galvanometer, 


Galvanometer  Shunts. 


39 

been  traversed  by  the  end  of  a pointer  whose  radius  was 
double  the  distance  from  the  mirror  to  the  scale. 

The  aperture  through  which  the  light  passes  is  sometimes 
a vertical  slit,  sometimes  a round  hole,  with  or  without  a 
a vertical  wire  stretched  across  it. 

Sometimes  the  mirror  is  plane,  and  the  light  is  brought 
to  a focus  on  the  scale  by  means  of  a lens.  Sometimes  the 
mirror  is  concave,  and  the  lens  is  dispensed  with. 

When  the  slit  is  used,  the  moving  image  is  a vertical  line 
of  light ; when  the  hole  is  used,  it  is  a bright  disc  crossed 
by  a fine  vertical  black  line,  the  image  of  the  wire. 

The  scale  is  usually  divided  into  millims.,  and  printed 
black  on  white  glazed  paper. 

In  using  a flat-wicked  paraffin-lamp,  the  wick  should  be 
placed  “ edgeways  that  is,  at  right  angles  to  the  scale. 

The  position  of  the  spot  of  light  on  the  scale  is  adjusted 
to  zero  by  the  curved  magnet  seen  at  the  top  of  Plate  III. 

It  has  a fine  and  coarse  circular  motion,  and  can  be  raised 
and  lowered  according  as  the  instrument  is  required  to  be 
more  or  less  sensitive. 

Galvanometer  Shunts. 

In  order  to  measure  other  than  very  feeble  currents  with 
these  galvanometers,  “ shunts  ” are  used  with  them,  i.e. 
resistances  so  proportioned  that  either  yoVo,  two-’  To>  or  the 
whole  current  to  be  measured,  can  be  sent  through  the 
galvanometer  at  will* 

Pig.  13  shows  such  a set.  The  wires 
bringing  the  current  are  attached  to 
the  two  binding  screws,  and  wires  are 
also  led  from  these  screws  to  the  ter- 
minals of  the  galvanometer. 

When  the  plug  is  in  the  hole  be- 
tween the  screws,  no  current  passes 
through  the  galvanometer. f When  it 
is  in  the  hole  marked  then 

of  the  whole  current  passes  through  the 

* See  page  24.  Each  galvanometer  must  have  its  own  shunts  ; a set 
made  for  one  instrument  cannot  be  used  with  another. 

f A plug  should  alwaj’s  be  kept  in  this  hole  when  the  instrument  is 
not  in  use. 


40 


Electric  Lighting . 

galvanometer ; when  in  and  when  in  T^-  passes 

respectively ; and  when  no  plug  is  in,  the  whole  current 
passes  through  the  galvanometer.  One  plug  only  is  used 
at  one  time. 

The  Tangent  Galvanometer. 

More  powerful  currents  may  be  measured  in  absolute 
units  by  means  of  a “ tangent 
galvanometer.”  The  form  of  tan- 
gent galvanometer  most  suitable 
to  this  purpose  consists  of  a single 
ring  of  wire  (fig.  14)  of  large 
diameter,  fixed  so  that  it  stands  in 
a vertical  plane,  and  having  a small 
compass  needle  at  its  centre.  To 
use  the  instrument  it  must  be 
turned  round  till  the  zero  of  the 
scale  is  opposite  the  point  of  the 
needle,  i.e.  until  the  ring  is  in  the 
magnetic  meridian. 

On  the  current  being  sent 
through  the  wire,  the  needle  will  be 
deflected.  When  we  know  the  diameter  of  the  ring  and  the 
strength  of  the  earth's  horizontal  magnetic  force,  we  can  cal- 
culate the  current  from  the  tangent  of  the  angle  of  deflection. 

The  tangent  of  an  angle  depends  only  on  the  angle,  and 
will  be  found  in  books  of  mathematical  tables,  and  in  the 
appendix  to  this  book. 

If  we  assume  that  for  England  the  earth  horizontal  force 
always  has  its  present  mean  value  at  Greenwich,*  we  shall 
not  introduce  a greater  error  than  others  inseparable  from 
the  construction  of  a single  ring  galvanometer. 

The  current  C in  Amperes  indicated  by  a deflection  of 
8 degrees,  when  the  ring  of  the  galvanometer  is  D inches 
in  diameter,  will  then  be  given  by  the  following  formula : — 
C = *362  D tan  5 (31)  t 

* H = *1794. 

f This  is  reduced  from  the  formula  given  in  my  “ Electricity,”  2nded. 
vol.  i.  pp.  247  and  259,  namely, 

C = H tan  *4- 

1 7 r 


Tangent  Galvanometer — Graduation . 41 

As  D is  the  same  for  all  experiments  with  the  same 
galvanometer,  it  will  save  trouble  if  the  value  of  ’362  P is 
calculated  for  each  particular  instrument,  and  marked  upon  it. 

For  instance,  if  the  ring  is  20  inches  diameter,  *362  D = 7*24, 
and  for  that  particular  instrument  the  formula  (31) 
becomes, — 

C = 7-24  tan  8. 

Problem : — 

What  current  is  indicated  by  a deflection  of  35°  when  the 
ring  is  one  foot  diameter  ? A reference  to  the  tables  gives 
us  tan  35°  = *7002,  and  from  (31)  we  have, — 

C = '362  x 1 2 x '7002  = 3*04  Amperes. 

If  the  ring  has  more  than  one  turn  of  wire,  the  number 
of  amperes  given  by  the  formula  must  be  divided  by  the 
number  of  turns  to  give  the  true  value  of  the  current,  or  in 
other  words  the  formula  (31)  becomes, — 

£ _ *362  D tan  8 
n 

Where  n is  the  number  of  turns. 

Another  method  of  graduating  a tangent  galvanometer 
requires  a knowledge  of  the  electro-motive  force  and 
internal  resistance  of  the  battery  used  to  deflect  it. 

This  method  is  not  so  accurate  as  the  first,  but  is  some- 
times useful  when  the  ring  cannot  be  accurately  measured. 

The  electro-motive  force  of  a Grove's  cell  has  a tolerably 
constant  value  of  1*93  volt. 

The  resistance  is  determined  as  follows  : — 

Let  a be  the  deflection  when  only  the  battery  and 
galvanometer  are  in  circuit.  The  current  is  proportional  to 
tan  a. 

Now  let  a small  known  resistance  r be  inserted.  The 
deflection  will  be  reduced  to  /?,  and  the  current  is  proportional 
to  tan  /?. 

where  H is  the  earth’s  horizontal  force  in  C.G.S.  measure,  a the  radius 
of  the  ring  in  centimetres,  and  tv  the  ratio  of  the  circumference  of  a circle 
to  its  diameter  = 31416. 


42 


Elective  Lighting. 


The  ratio  of  the  two  currents  is, — 

tan  a 
tan  /3. 

But  the  electro-motive  forces  being  the  same,*  the  ratio  of 
the  currents  is  the  inverse  ratio  of  the  total  resistances  in 
circuit  in  the  two  cases. 

Let  x be  the  resistance  of  the  battery,  galvanometer,  and 
connecting  wires. 

In  the  first  case,  when  the  deflection  was  a,  x was  the  total 
resistance  in  circuit.  In  the  second,  where  the  deflection 
was  /3,  the  total  resistance  was  x + r. 

The  ratio  of  the  currents  was  therefore, — 


and  we  have, — 


or,— 


X + V 

X. 

tan  a x + r 

tan  (3  x. 

tan  a .x  = tan  (3  (x  -f-  r). 


or,- 


or. 


(tan  a — tan  /3)  x = tan  /3  . r. 
tan  (3 


tan  a — tan  (3 


(33) 


Having  thus  obtained  the  total  resistance  in  circuit,  we 
can  calculate  the  value  of  the  deflection  in  amperes,  by 
removing  the  resistance  r,  and  sending  the  current  from 
several  cells  through  the  galvanometer  and  using  the 
formula, — 


Where  n is  the  number  of  cells,  and  e the  E.M.F.  of  one 
cell.  For  a Grove's  cell  e — 1*93  volt,  approximately. 

Let  8 be  the  deflection  given  by  the  now  known  current  C. 
We  have, — 


C = k tan  d . 
whence — 

^ tan  8 


(35) 
. (36) 


# The  electro-motive  force  is  apt  to  vary  a little  with  change  of  resist- 
ance, and  hence  the  method  is  not  perfect. 


Graduation  of  Tangent  Galvanometer . 43 

&,  being  the  constant  of  the  galvanometer,  tbe  same 
quantity  as  the  formula  (32)  gave  equal  to, — 

•362  D 

n. 

Example. 

Let  us  suppose  we  have  a galvanometer  given  us  to 
graduate,  and  that  we  use  4 Grove’s  cells. 

We  have  first  to  obtain  the  resistance  x (eq.  33)  of  the 
battery,  galvanometer,  and  connecting  wires. 

Suppose  the  deflection  to  be  40°  when  the  cells  are  con- 
nected direct  to  the  galvanometer. 

We  have  a = 40°,  and  from  the  tables,  tan  a = *83909. 
We  now  insert  ^ ohm  resistance,  the  deflection  will  be 
reduced,  say  to  29°. 

We  have  /3  = 29°,  and  from  the  tables,  tan  /3  = *55430. 
From  (33)  we  have, — 

•55430  , 1 ah  1 

^'= -83909- -55430  X*  = 1'47ohm- 


To  find  the  constant  of  the  galvanometer  we  have  from 

(34), — 


4 x 1*93 

— 1-47. 


when  no  extra  resistance  is  inserted. 

But  we  also  found  when  no  resistance  was  inserted,  the 
deflection  8 was  40°,  and  tan  8 = *83909. 

Hence  we  have  using  (36), — 


4 x 1*93 
•83909  x 1-47 


6-27. 


And  the  formula  (35)  for  this  particular  galvanometer 
becomes.— 

C (in  amperes)  = 6'27  tan  8. 

If  a galvanometer  is  much  used,  it  is  convenient  to 
calculate  the  value  of  h tan  8 for  each  degree,  and  use  the 
table  so  prepared,  instead  of  making  a fresh  calculation  for 
each  experiment. 

Currents  exceeding  10  amperes  may  be  measured  by 
using  a shunt  (p.  25,  fig.  7,  and  p.  39,  fig.  13)  and  sending 
a known  fraction  of  the  current  through  the  galvanometer. 


44 


Electric  Lighting . 


Professors  Ayrton  and  Perry's  Instruments. 

Professors  Ayrton  and  Perry  have  devised  a series  of 
instruments  specially  adapted  for  making  the  electrical 
measurements  required  in  electric  lighting.  The  conditions 
which  they  have  had  in  their  minds  in  devising  them  have 
been  the  following  : — 

First,  the  instruments  must  be  portable,  must  be  mode- 
rately cheap,  and  easy  to  use. 

Second,  and  this  is  most  important,  they  must  be  “ dead- 
beat " — i.e.  changes  in  the  quantity  which  is  being  measured 
must  be  instantly  indicated  by  the  needle  without  any 
oscillation. 

Third,  it  must  be  easy  to  calibrate  them,  and  to  verify  the 
calibration  at  any  future  time. 

The  Ammeter. 

The  instrument  they  use  for  the  measurement  of  currents 
is  called  the  Ammeter , or  ampere-measurer,  and  is  a special 
form  of  galvanometer.  It  is  shown  at  about  three-quarters 
its  actual  size  in  fig.  15. 

The  case  of  the  instrument  is  composed  of  a powerful 
horseshoe  magnet,  inside  which  the  needle  moves  on  a 
vertical  pivot.  A pointer  attached  to  the  needle  allows  the 
deflection  to  be  read  on  the  scale  on  the  top  of  the  instru- 
ment. The  horseshoe  magnet  so  acts  on  the  needle  as  to 
always  tend  to  bring  back  the  pointer  to  zero.  It  gives  a 
constant  magnetic  field  independent  of  changes  in  the 
earth's  magnetism.  The  needle  is  deflected  by  a coil  of 
wire  consisting  of  ten  rings  surrounding  it. 

These  rings  are  attached  to  springs  which  press  on  the 
roller  seen  on  the  right.  When  the  roller  is  turned  in  one 
direction  all  the  rings  are  connected  in  series,  so  that  a 
current  entering  the  instrument  goes  round  them  one  after 
another,  and  so  passes  ten  times  round  the  needle.  When 
the  roller  is  turned  in  the  other  direction,  marked  “ quan- 
tity," the  wires  are  all  connected  laterally,  so  that  they 
are  equivalent  to  one  very  thick  wire,  and  the  current 
passes  only  once  round  the  needle. 


45 


Ayrton  and  Perry  s Ammeter. 

We  s§e  tliat  with  the  series 33  arrangement  a given 
current  has  ten  times  the  effect  on  the  needle  that  it  would 
have  if  the  roller  was  turned  to  “ quantity.”  We  therefore 
use  the  series  arrangement  for  measuring  feeble  currents, 
and  the  quantity  arrangement  for  measuring  strong  ones. 

The  coils  and  the  inside  shape  of  the  poles  are  so  arranged 
that  the  deflection  is  proportional  to  the  current. 

The  needle  is  deflected  right  or  left  according  to  the 


Fig.  15. 


direction  of  the  current,  and  it  can  move  through  45°  in 
each  direction. 

The  roller  arrangement  allows  the  instrument  to  be 
easily  graduated  by  means  of  a few  cells  of  a battery  of 
known  electro-motive  force ; as  when  the  roller  is  placed 
in  the  “ series  ” position  the  current  of  three  or  four  cells 
produces  a considerable  deflection.  Its  indications  may 
either  be  compared  with  those  of  a measured  tangent  gal- 
vanometer (eq.  32,  page  41),  or  we  may  use  the  method  of 


46 


Electric  Lighting . 

graduating  described  on  pages  41 — 43.  The  little  plug  in 
the  front  right-hand  corner  of  fig.  15  short-circuits  a resist- 
ance coil  of  one  ohm— so  that  when  the  plug  is  removed 
one  ohm  resistance  is  added  to  the  circuit. 

To  determine  the  resistance  of  the  battery  and  galva- 
nometer we  use  formula  (33),  only  instead  of  tan  a and  tan 
we  use  the  deflections  a and  /3  themselves,  as  in  this 
instrument  the  currents  are  proportional  to  the  deflections 
and  not  to  their  tangents. 

We  also  remember  that  r — 1. 

The  formula  (33)  then  becomes 


a ■ 


0 


(37) 


To  obtain  the  value  of  the  current  we  still  use  the 
formula — - 


C=^ 

x 


(34) 


while  to  calculate  the  constant  of  the  instrument,  (36) 
becomes 


k-C 

d 


(38) 


and  the  formula  for  using  the  instrument  is  altered  from 
(35)  to 


C = k 


(39) 


Messrs.  Ayrton  and  Perry  now  usually  adjust  their  instru- 
ments so  that  k s=  -l  when  the  roller  is  at  series,  and  2 when 
it  is  at  quantity. 

k = means  that  each  degree  is  ampere,  or  that  an 
ampere  deflects  5°  while  k = 2 means  that  each  degree  is 
2 amperes,  or  that  an  ampere  deflects  J°. 

Problem  : — 

(1)  With  an  instrument  so  adjusted,  what  current  is  indi- 
cated by  a deflection  of  15°  when  the  roller  is  at  series  ? 

(39)  gives  us 

C = ^ x 15  = 3 amperes. 


Thomson' s Graded  Galvanometer.  47 

(2)  What  current  will  the  same  deflection  indicate  when  the 
roller  is  at  quantity  ? 

C = 2 x 15  = 30  amperes. 

The  instrument  is  absolutely  dead-beat,  and  the  shortest 
variation  of  the  current  is  shown  on  it. 

For  instance,  if  a light  dynamo  machine  is  being  driven 
by  belting,  the  needle  of  the  ammeter  gives  a little  jump 
each  time  the  lacing  of  the  belt  passes  the  pulley. 

We  see  then  that  with  the  series  arrangement  we  can 
measure  currents  up  to  9 amperes,  and  with  the  quantity 
arrangement  up  to  90  amperes. 

When  used  for  measuring  strong  currents  by  the  quantity 
arrangement,  the  wires  are  to  be  attached  to  the  screws 
P P ; for  weak  currents  with  the  series  arrangement,  they 
are  to  be  attached  to  S S.  The  instrument  is  so  arranged 
that  it  is  impossible  to  send  a strong  current  through  the 
series  arrangements  as  long  as  P and  S are  not  interchanged. 

When  the  instrument  is  not  in  use,  the  poles  of  the 
magnet  are  connected  by  a soft  iron  armature.  In  order 
that  the  observer  may  not  forget  to  take  off  the  armature  on 
commencing  work,  a brass  cover  for  the  dial  may  be  con- 
veniently fixed  to  the  armature,  so  that  the  dial  cannot  be 
seen  except  when  the  armature  is  removed. 

Sir  Wm.  Thomson's  Graded  Galvanometer  (Fig.  16). 

This  is  an  absolute  galvanometer,  with  a very  wide  range  of 
usefulness,  as  the  magnet  and  scale  can  be  moved  nearer  or 
further  from  the  coil,  according  to  the  strength  of  the  cur- 
rent under  examination.  With  a feeble  current  the  magnet 
is  placed  close  to  the  coil,  and  a good  deflection  is  thus 
obtained,  while  with  more  powerful  currents  it  is  moved 
further  off,  and  the  deflection  is  still  kept  within  the  range 
of  the  instrument.  Numbers  are  engraved  on  the  scale 
along  which  the  magnet-holder  slides,  which  are  used  in 
calculating  the  value  of  the  current  corresponding  to  a 
given  deflection. 

The  needle  is  brought  to  zero  partly  by  the  earth's  force 
and  partly  by  the  curved  magnet  shown  in  fig.  16. 


48 


Electric  Lighting. 

In  adjusting  the  instrument,  the  magnet  must  be  re- 
moved, and  the  apparatus  turned  until  the  needle  is  at  zero 
under  the  influence  of  the  earth's  magnetism,  i.e.  until  it 
lies  in  the  meridian.  The  magnet  must  then  be  replaced 
and  adjusted  by  its  screw  until  the  pointer  is  again  at  zero. 
The  magnet-holder  should  be  slid  along  the  scale  and 
stopped  at  some  exact  division  which  will  make  the  deflec  - 
tion  somewhere  between  15°  and  40°. 


Fig.  16. 


To  find  the  number  of  amperes  corresponding  to  a deflec- 
tion— 

Rule. — Multiply  the  number  of  divisions  in  the  deflection 
by  the  number  on  the  magnet,  plus*  T7  for  the  horizontal 
intensity  of  the  earth9 s field,  and  divide  by  the  number  at  the 
division  on  the  platform  scale  exactly  under  the  front  of  the 
magnetometer. 

In  other  words,  if  we  call  the  scale  number  (i.e.  the 


* The  earth’s  horizontal  magnetic  force  varies  in  different  localities! 
The  following  table  gives  the  number  which  must  be  added  to  the  number 


in  the  curved  magnet  when  great 

accuracy  is  required. 

For  ordinary 

work  the  number 

■17  may  be  used. 

Cambridge 

. 181 

Newcastle  . 

. 167 

London 

. *181 

Dublin 

. -167 

Birmingham 

. T 76 

Carlisle 

. -166 

Nottingham 

. T75 

Edinburgh  . 

. *162 

Stafford 

. 475 

Glasgow 

. 461 

Sheffield  . 

. T73 

Dundee 

. *161 

Manchester 

. *172 

Aberdeen  . 

. T58 

Liverpool  . 

. -171 

Inverness  . 

. *156 

York  . 

. *171 

49 


Measurement  of  Electro-motive  Force. 


number  on  the  base  at  the  front  of  the  magnet  and  scale) 
S,  tbe  strength  of  the  magnet  M,  and  the  deflection  8,  then 
the  current  0 will  be  given  by  the  formula, — 

C = - (M  + -1- (40) 

Example. 

If  the  strength  of  tbe  magnet  is  12§24,  and  the  magnet 
and  scale  are  placed  at  the  mark  3*1  on  the  base  of  the 
instrument,  what  current  is  indicated  by  a deflection  of  11°  ? 

We  have  from  (40), — 


C = 


11  x (12*24+  -17) 
31 


= 44-05  amperes. 


Terminal  pieces  of  the  form  shown  in  fig.  16  are  attached 
to  the  coil  of  the  instrument,  and  to  the  electrodes  supplied 
with  it.  When  the  electrodes  are  being  removed  from  the 
coil  or  from  the  leads,  the  two  sides  of  the  spring  terminal 
piece  should  come  into  contact  with  each  other  before  they 
are  out  of  contact  with  the  plates  of  the  other  terminal  pieces. 
By  attending  to  this  the  circuit  is  not  interrupted,  and 
hence  sparks  are  avoided.  A separate  terminal  piece, 
shown  in  the  figure  with  two  short  wires  attached,  is  also 
supplied,  for  the  purpose  of  allowing  the  galvanometer  to 
be  easily  introduced  or  removed  from  the  circuit.  This 
terminal  piece  is  made  to  form  part  of  the  circuit  the 
current  through  which  is  to  be  measured.  By  adopting 
this  arrangement  the  galvanometer  can  be  neadily  removed 
from  one  circuit  to  another. 


Electro-motive  Force. 


If  the  wires  of  a galvanometer  whose  deflection  is  pro- 
portional to  the 
current  passing 
through  it,  be 
attached  to  any 
two  points  in 
a circuit — as  for 
instance,  to  the 
wires  on  the 
two  sides  of  a 
lamp,  as  in  fig. 


Fig.  17. 


E 


50 


Electric  Lighting. 

17 — its  deflection  will  be  proportional  to  the  electro-motive 
force  between  those  two  points  after  the  attachment  of  the 
galvanometer.  If  the  resistance  of  the  galvanometer  is 
very  great  compared  to  that  of  the  lamp,  its  attachment 
will  not  perceptibly  disturb  the  electro-motive  force,  and 
therefore  the  deflection  will  be  a measure  of  the  electro- 
motive force  which  is  driving  the  current  through  the  lamp. 

For  let  R,  be  the  resistance  of  the  galvanometer,  E the 
electro-motive  force  between  the  two  points  where  its  wires 
are  attached,  and  C the  current  through  the  galvanometer, 
then  we  have  from  Ohm’s  law  (2),  page  12,— 

E = C R. 

In  order  to  make  this  experiment,  it  is  of  course  necessary 
to  know  what  current  in  amperes  is  indicated  by  a given 
deflection  of  the  needle. 

The  Voltmeter  (Ayrton  and  Perry’s  Pattern). 

One  of  the  various  Voltmeters  used  for  measuring  electro- 
motive forces  on  this  principle  is  that  devised  by  Professors 
Ayrton  and  Perry.  In  appearance  it  is  precisely  similar  to 
the  ammeter  (fig.  15),  but  its  coil  consists  of  a number 
of  turns  of  very  fine  wire. 

When  the  roller  is  placed  at  “ series,”  the  current  goes 
10  times  as  often  round  the  needle,  and  the  resistance  is 
100  times  as  great  as  when  the  roller  is  at  “ quantity.” 

The  calibration  is  performed  in  precisely  the  same  way  as 
the  calibration  of  the  ammeter,  except  that  in  the  present 
case  the  roller  is  placed  at  f<r  quantity  ” for  the  calibration 
and  at  “ series  ” for  the  actual  work,  as  we  wish  in  work  to 
have  as  high  a resistance  as  possible,  and  in  calibration  to 
send  a sufficient  current  through  the  instrument  with  a 
moderate  number  of  cells. 

Sir  Wm.  Thomson’s  Voltmeter  or  Potential 
Galvanometer. 

This  instrument  is  adjusted  and  used  in  the  same  way  as 
the  ammeter  or  current-galvanometer  described  on  page  47. 

It  is  shown  in  fig.  18,  and  consists  essentially  of  a coil 


Thomson' s Voltmeter . 


5i 


of  insulated  copper  or  German  silver  wire  C,  the  resistance 
of  which  is  generally  over  5000  ohms,  fixed  to  one  end  of 
a platform  P,  on  which  a magnetometer  M rests. 

In  order  to  facilitate  the  use  of  the  instrument,  a pair  of 
flexible  electrodes,  about  4 yards  long,  are  supplied  along 
with  it.  These  electrodes  are  shown  attached  to  the  instru- 
ment in  the  fig.  (18).  The  spring  clips  attached  to  the  ends 
of  the  electrodes  allow  the  instrument  to  be  readily  put  in 


Fig.  18. 


contact  with  two  points  of  a circuit.  To  prevent  a current 
passing  through  the  coil  when  no  reading  is  being  taken, 
a spring  key  is  placed  in  the  circuit  of  the  coil.  This  key 
should  on  no  account  be  held  long  in  contact,  because  the 
coil  becomes  heated  when  a current  is  allowed  to  flow  con- 
tinuously through  it,  and  consequently  increases  in  resist- 
ance, thus  causing  the  indications  to  be  too  small. 

To  determine  the  difference  of  potential  between  two 
points  of  a circuit,  an  electrode  is  clipped  on  at  each  of  the 
points  and  then  the  key  depressed  and  the  deflection  noted. 
If  the  deflection  be  too  great,  the  magnetometer  must  be 
pushed  to  a division  further  from  the  coil ; if  too  small,  to  a 
division  nearer  the  coil.  The  number  of  divisions  in  the 
deflection  is  then  to  be  multiplied  by  the  number  on  the 
magnets,  plus,  say  T7*  for  the  earth’s  force,  and  divided  by 
the  number  at  the  division  of  the  scale  on  the  platform  exactly 
under  the  front  of  the  magnetometer  ; the  result  is  the  difference 
of  the  potential  in  volts. 

In  other  words,  if  we  call  S the  scale  number,  M the 
strength  of  the  magnet,  and  8 the  deflection  (as  in  (40)),  the 

* See  footnote,  page  48. 

E 2 


52 


Electric  Lighting . 


E.M.F.,  E in  volts,  corresponding  to  any  deflection  will  be 
given  by  tbe  formula, — 


E = * (M  + -17) 


• (41) 


Example. 

The  magnetometer  is  at  the  division  marked  1*8,  the 
strength  of  the  curved  magnet  (marked  on  it)  is  11*37,  the 
deflection  is  18° ; what  is  the  E.M.F.  ? 

We  have  from  (41), — 


E 


18  (11-37  + -17) 
1-8 


115-4  volts. 


When  the  difference  of  potential  to  be  measured  exceeds 
200  volts,  the  readings  of  deflection  must  be  taken  as  quickly 
as  possible  on  account  of  the  rapid  heating  of  the  coil.* 

The  Caedew  Voltmeter. 

Just  as  these  sheets  are  going  to  press  I have  had  an 
opportunity  of  examining  an  admirable  voltmeter  invented 
by  Mr.  Cardew,  which  is  equally  useful  for  direct  and  for 
alternating  currents,  and  which  appears  to  be  very  accurate, 
and  which  certainly  is  very  sensitive. 

It  consists  of  a long,  fine  platinum- silver  wire  of  high 
resistance  (about  7 or  8 feet  long,  and  *003  inch  in  diameter), 
one  end  of  which  is  rigidly  fixed,  and  the  other  is  kept  tight 
by  a spring,  and  attached  to  the  axle  of  an  indicating  hand. 
The  ends  of  the  wire  are  connected  to  the  poles.  The  current 
heats  and  expands  the  wire,  and  the  amount  of  expansion  is 
shown  on  the  dial.  The  instrument  is  so  sensitive,  that  if  a 
dynamo  is  being  driven  by  a small  engine,  every  stroke  of 
the  engine  is  indicated  by  a quiver  of  the  pointer. 


Kesistance. 


When  feeble  currents  can  be  used,  and  when  the  resistance 
under  examination  is  constant,  i.e.  does  not  alter  with  the 
current,  the  best  method  of  measuring  it  is  that  known  as 
Wheatstone's  bridge."  f 


* Tables  of  corrections  to  apply  for  the  heating  are  supplied  with  the 
instruments,  hut  are  not  much  used. 

f Let  r he  the  resistance  of  each  of  the  10  coils.  When  they  are  in 
series  the  resistance  is  (from  19)  10  r ; when  they  are  in  quantity  the 
resistance  is  (from  18)  Tg  r. 


53 


Resistance . — Wheatstone' s Bridge . 

The  theory  of  it  is  given  in  my  Electricity.”  * 

The  following  are  practi- 
cal rules  for  its  use. 

Fig.  19  is  a diagram  of 
the  connections. 

Three  known  resistances 
and  the  resistance  x to  be 
measured,  are  connected  so 
as  to  form  the  four  sides  of 
a square.  Two  diagonally 
opposite  corners  of  the 
square  are  connected  to  the 
poles  of  a battery,  the  other  two  to  those  of  a sensitive 
galvanometer.  It  is  indifferent  to  which  pair  each  is  con- 
nected. 

The  resistance  R is  then  varied,  and  wTe  shall  find  that  at 
one  particular  value  of  R there  is  no  deflection  of  the 
galvanometer. 

When  this  is  the  case,  the  products  of  the  resistances  of 
opposite  sides  of  the  square  are  equal , or 

S*  = sU  . . . . . (42) 

whence 


Fig.  19. 


* = |e (43) 

The  relative  values  which  we  must  give  to  s and  S are 
determined  by  the  range  through  which  we  can  vary  R,  and 
by  the  relative  magnitudes  of  R and  x. 

In  an  ordinary  “ resistance-box 3)  R can  be  varied  from 
1 ohm  to  10,000  ohms,  and  s and  S can  each  be  made  either 
10,  100,  or  1000  ohms. 

If  we  knew  that  x was  under  10  ohms,  we  should  pro- 
bably put  s = 10  and  S = 1000.  If  then  we  found  there 
was  no  deflection  when.  R was  721  ohms,  we  should  know 
that  our  resistance  was 

* = M5o721  = 7'21  ohms> 

and  we  have  thus  measured  x to  T^-  of  an  ohm. 


* 2nd  edition,  vol.  i.  p.  247. 


54 


Electric  Lighting . 

If  x was  6000  or  7000  ohms,  we  should  probably  put 
S = 1000,  s = 1000;  and  then  if  we  found  R = 6475  we 
should  have 


1000^^  . 
x — 2qqq  6475  = 6475  ohm?. 


Lastly,  if  x were  over  half  a million  ohms,  we  should  put 
S =10,  s = 1000  ; and  suppose  R were  8452,  we  should 
have 

* = ^ 8452  = 845,200  ohms. 


We  see,  therefore,  with  coils  such  as  we  have  mentioned 
we  can  measure  from  y-Jy  ohm  to  1,000,000  ohms. 

In  practice  the  Bridge  shape  is  not  used,  but  the  resist- 
ances are  arranged 
in  a “ resistance- 
box”  (fig.  20).  On 
the  lid  of  the  box 
are  brass  blocks  of 
the  shape  shown  in 
plan  in  fig.  21. 
These  can  be  con- 
nected by  brass  plugs  inserted  at  the  points  a,  b - - -. 

The  resistance-coils  in  the 
box  are  connected  to  the 
brass  blocks  in  the  manner 
Flg'21*  shown  in  fig.  22.  Thus, 

when  the  plugs  are  inserted  the  current  passing  through 
them  meets  with  no  resistance.  When  any  one  is  removed, 


Fig.  20. 


A 


BQDm 


i r 


l the  current  has  to  pass 
Z)B  through  the  resistance- 
coil  under  it. 

The  value  of  the  re- 
Fig.  22.  sistance  at  each  opening 

is  marked  on  the  lid  of  the  box,  as  shown  in  fig.  23.  Thus 
the  resistance  unplugged  is  the  resistance  in  circuit.  Fig. 
23  represents  a resistance-box  arranged  for  bridge  measure- 


Resistance  with  Strong  Currents . 55 

ments.  The  arrangement  is  precisely  similar  electrically  to 


that  shown  in  diagram  in  fig.  19,  as  may  be  seen  on 
comparison  of  the  two  figs.,  noting  that  the  same  letters  are 
used  for  the  same  parts  in  both. 

The  arrangement  of  the  resistance  R should  be  noted, 
by  which,  with  only  sixteen  coils,  any  resistance  from  1 ohm 
to  10,000  ohms  can  be  inserted. 

In  making  up  any  required  resistance,  the  largest  number 
possible  should  be  used  first.  As  fig.  23  is  drawn,  we  have 
5 = 10,  S = 1000,  and  R=2163,  whence  when  the  galvano- 
meter is  unaffected 

x — 21  ‘63  ohms. 

Resistance  with  Strong  Currents. 

When,  as  in  the  case  of  electric  light  measurements, 
the  passage  of  the  current  considerably  alters  the  re- 
sistance, it  is  necessary  to  use  some  method  by  which  the 
resistance  can  be  measured  while  a powerful  current  is  flow- 
ing through  it,  and  in  which  it  is  not  necessary  to  start  and 
stop  the  current  in  the  course  of  the  observations. 

Resistance  can  be  determined  by  simultaneous  observa- 
tions with  the  ammeter,  or  with  a tangent  galvanometer,  and 
with  the  voltmeter  or  other  suitable  high  resistance  gal- 
vanometer, for  the  ratio  of  the  two  quantities  determined  is 
the  resistance  required.  Thus  if  E be  the  E.M.F.  in  volts, 
between  two  points  in  the  circuit,  and  C the  whole  current 
in  amperes,  the  resistance  between  these  points  is  given  by 
the  equation, — 


56 


Electric  Lighting. 


• (3)* 


This  method  is  particularly  useful  for  the  measurement  of 
the  resistances  of  electric  lamps,  which  are  quite  different 
when  they  are  hot  and  when  they  are  cold. 

Problem. 

A certain  incandescent  lamp  requires  42  volts,  to  send  a 
current  of  1 *4  ampere  through  it,  what  is  its  resistance  with 
that  current  ? 

We  have  from  (3), — 

E = = 30  ohms. 

14 


The  Ohmmetee. 

Professors  Ayrton  and  Perry  have  arranged  an  instrument 
which  they  call  the  Ohmmeter  (fig.  24),  in  which  the  needle  is 


Fig.  21. 


deflected  by  a coil  carrying  the  main  current,  and  so  corre- 
sponding to  the  coil  of  the  ammeter,  but  is  brought  back  to 
zero,  not  by  a constant  permanent  magnet,  but  by  an  electro- 
magnet wound  with  fine  wire,  and  connected  in  the  same 
manner  as  the  coil  of  the  voltmeter.  The  amount  of  deflec- 
tion depends  on  the  ratio  of  the  currents  in  the  magnet  and 

* Page  12 


The  Ohmmeter. 


57 


in  the  coil  respectively,  that  is,  on  the  ratio  of  the  E.M.F. 
to  the  current,  and  so  measures  the  resistance  between  the 
two  points  where  the  ends  of  the  magnet  wire  are  attached. 
By  properly  adjusting  the  shape  of  the  pole  pieces  and  the 
position  of  the  coils,  the  deflections  of  the  instrument  are 
made  proportional  to  the  resistance. 

Fig.  25  shows  the  connections. 

As  at  present  constructed  the  magnet  wire  has  about 


B’i^.  25. 


700  ohms  resistance,  and  the  deflecting  coil  about  *003 
ohm. 

It  is  found  that  owing  to  the  residual  magnetism  the 
needle  does  not  return  to  zero  between  different  observa- 
tions, but  remains  in  a position  depending  on  the  Jast 
resistance  observed.  This,  however,  does  not  affect  the 
accuracy  of  the  observations.  The  readings  are  to  be  taken 
from  the  zero  marked  on  the  scale,  and  not  from  the  position 
of  rest. 

Thus  if  the  needle  indicates  20°  in  a particular  observa- 
tion, we  know  that  the  resistance  under  examination  is  that 
corresponding  to  20°  deflection,  whether  the  needle  before 
the  observation  rested  at  zero  or  at  10°  or  at  30°. 


58 


Elective  Lighting. 


Horse-power  expended  in  a Lamp  or  any  other  portion 
oe  the  Circuit. 

This  can  also  be  determined  by  the  use  of  the  ammeter 
and  voltmeter  j for  when  we  know  E and  C;  we  know  the 


Fig.  26. 


horse-power  from  the  equation, — 


H.P. 


EC 
746  * 


(7)* 


The  Electric-power-meter. 

Professors  Ayrton  and  Perry  have  arranged  an  instru- 
* Page  15. 


59 


The  Elective  Horse-power  Meier . 

ment  (fig.  26)  in  which  the  needle  shows  the  amount  of 
attraction  between  a fixed  coil  carrying  the  main  current 
and  a suspended  coil  of  high  resistance,  which  is  connected 
to  the  two  sides  of  the  lamp  like  the  coil  of  the  voltmeter. 
The  attraction  between  the  two  coils  is  proportional  to  the 
product  of  the  currents  in  them,  or  to  the  product  of  the 
main  current  into  the  E.M.F,,  that  is  to  the  horse-power. 

The  fixed  coil  is  wound  with  10  turns  of  wire,  which  can 
be  connected,  either  in  quantity  or  series,  by  means  of  a 
roller  exactly  like  that  used  in  the  ammeter  and  voltmeter. 

The  fine  wire  coil  has  700  ohms  resistance,  and  is  placed 
vertically  in  the  centre  of  the  fixed  coil.  It  turns  freely  on 
a jewelled  pivot. 

In  some  of  the  instruments  a cog-wheel  on  the  axis  gears 
into  another  carrying  an  aluminium  pointer  so  as  to  mul- 
tiply the  angle  of  deflection  6 times.  The  pointer  is  brought 
back  to  zero  by  means  of  two  spiral  springs  like  those  used 
in  chronometers,  one  on  the  axis  of  the  turning  coil  and  one 
on  the  axis  carrying  the  pointer.  By  means  of  a lever  one 
of  these  springs  can  be  detached  so  as  to  increase  the  sen- 
sitiveness of  the  instrument. 

The  total  range  of  the  pointer  is  270°.  When  the  coils 
are  in  series  and  only  one  spring  connected,  TV  H.P.  moves 
the  needle  over  the  whole  scale,  or  1°  corresponds  to 
2--7V0  H.P.  When  the  coils  are  in  quantity  and  both  springs 
connected,  a deflection  of  270°  corresponds  to  6 H.P. 

We  see,  therefore,  that  the  instrument  will  indicate  from 
2T00  H.P.  t0  6 H.P. 

The  instrument  is  graduated  in  exactly  the  same  manner 
as  the  ammeter,  by  means  of  the  roller  and  the  resistance- 
coil,  whose  plug  is  seen  at  the  left  front  part  of  fig.  26. 

Thus  to  sum  up, — 

The  Ammeter  measures  C. 

The  Voltmeter  measures  E. 

E 

The  Ohmmeter  measures  the  ratio  ^ = R.* 

The  Eiectric-power-meter  measures  the  product  EC  = 746  HP.* 

* I have  no  experience  of  the  practical  working  of  these  two  instru- 
ments. 


60  Electric  Lighting. 

We  again  note,  that  by  means  of  the  first  two  instruments 
it  is  possible  to  calculate  the  last  two  quantities,  if  the 
measurements  are  made  simultaneously. 

Measurement  of  Alternating  Currents. 

The  strength  in  amperes  of  an  alternating  current  can 
be  measured  by  the  Siemens  dynamometer  (page  35).  This 
instrument  is  excellent  for  measuring  the  powerful  currents 
in  arc  lamps  or  in  groups  of  incandescent  lamps,  but  it  is 
not  quite  so  satisfactory  when  used  for  the  measurement  of 
the  currents  under  2 amperes  used  by  single  incandescent 
lamps. 

The  mean  E.M.F.  of  an  alternating  current  can  be 
measured  by  means  of  the  Caraew  voltmeter,  or  an  electro- 
dynamometer of  high  resistance  might  be  used,  in  the  same 
manner  as  the  voltmeter. 

For  measuring  the  H.P.  expended  in  any  part  of  the 
circuit,  Ayrton  and  Perry’s  Electric-power-meter  might  be 
used  : owing,  however,  to  self-induction,*  neither  the  power- 
meter  nor  the  high  resistance  dynamometer  are  quite 
satisfactory  .f 

In  my  own  experience  I have  found  that  a useful 
way  to  measure  the  electric  current,  electro-motive  force, 
or  horse-power  used  in  an  incandescent  lamp  by  an 
alternating  current,  is  to  note  the  candle-power  exactly,  and 
then  to  bring  the  same  lamp  to  the  same  brightness  by  a 
Grove  battery  or  by  a direct-current  machine,  and  to  make 
the  necessary  measurements  on  the  direct  current  by  the 
methods  already  described. 

The  battery  is  preferable  to  the  machine  for  some  reasons, 
but  the  machine  has  the  advantage  of  being  always  ready 
in  the  factory,  and  can  be  started  by  merely  pulling  a lever, 
whereas  30  or  40  Groves  cells  take  perhaps  an  hour  to  set 
up.  With  the  battery  the  current  is  regulated  by  altering 
the  number  of  cells ; with  a machine  it  is  adjusted  by  intro- 
ducing different  resistances,  or  by  varying  the  speed,  if  the 
machine  is  driven  by  an  independent  engine. 

# See  Chapter  IX. 

f The  Cardew  voltmeter  is  practically  not  affected  by  self-induction. 


CHAPTER  VI. 


INCANDESCENT  LAMPS. 

The  class  of  lamps  known  as  t£  incandescent  ” consists 
of  a tkin  filament  or  wire  of  carbon  enclosed  in  a glass 
globe  from  wbicli  the  air  has  been  exhausted. 

On  a suitable  current  of  electricity  being  sent  through 
the  filament  it  becomes  white  hot,  or  incandescent,  and 
gives  a light  of  from  1 to  100  candles  according  to  its 
surface,  and  for  a given  surface  according  to  the  tempera- 
ture to  which  it  is  raised.  For  a given  temperature  the 
durability  of  the  filament  depends  on  its  uniformity,  and  on 
the  completeness  with  which  the  air  has  been  exhausted. 
Below  a certain  temperature,  nearly  corresponding  to  that 
of  melting  platinum,  a well-made  filament  in  a good 
vacuum  is  very  durable.  Under  these  conditions,  lamps  last 
six  or  twelve  months  of  ordinary  domestic  work. 

The  chief  incandescent  lamps  now  in  actual  use  are  the 
Swan,  Edison,  Maxim,  Lane-Fox,  Woodhouse  and  Rawson, 
and  Crookes.  They  differ  from  each  other  in  the  methods 
of  preparing  the  carbon  filaments  and  in  other  details. 

Each  inventor  has  patented  his  processes  of  manufacture, 
but  I shall  purposely  abstain  from  expressing  any  opinion 
as  to  the  validity  of  some  of  the  patents. 

I propose  to  give  a general  description  of  one  or  two  of 
the  lamps  which  may  be  considered  as  typical  ones,  and  to 
describe  some  of  the  processes  used  in  their  manufacture. 

I shall,  however,  avoid  technical  details  as  much  as 
possible,  partly  because  I have  not  practical  experience  of 
the  processes,  and  partly  because  it  is  not  so  much  impor- 
tant for  electrical  engineers  to  be  prepared  to  manufacture 


62  E lectric  L igh  ting. 

lamps,  as  to  be  able  to  erect  and  manage  them  when  they 
are  supplied  by  the  present  manufacturers. 

The  first  public  exhibition  of  incandescent  lamps  that  was 
made  in  this  country  was  made  by  Mr.  Swan  before  the 
Society  of  Telegraph  Engineers  on  November  24,  1880. 
The  first  exhibition  in  America  was  made  by  Mr.  Edison. 

Incandescent  Lamps. — The  Caebon. 

In  all  incandescent  lamps  the  filament  consists  of  a thread 
of  some  vegetable  substance  which  has  been  carbonized 
by  heat. 

The  Terminals. 

The  ends  of  the  filaments  are  connected  to  two  plati- 
num wires,  which  pass  through  the  glass  and  are  melted 
on  to  it.  Platinum  is  used  as,  its  expansion  rate  being 
about  the  same  as  that  of  glass,  the  latter  does  not 
crack  in  cooling. 

The  Exhaustion. 

The  life  of  the  lamp  depends  in  a large  measure  on  the 
goodness  of  the  vacuum.  In  order  to  get  a good  vacuum, 
various  modifications  of  the  Sprengel  mercury-pump  have 
been  made,  all  having  for  their  object  to  fit  an  instrument 
hitherto  only  used  in  laboratories  to  the  more  rapid  pro- 
cesses of  the  factory. 

Hot  Exhaustion. 

It  was  soon  found  that  however  perfectly  the  lamp  was 
exhausted  when  cold,  yet  that  the  first  time  a current  was 
sent  through  it  a quantity  of  gas  was  driven  out  of  the  car- 
bon itself,  which  injured  the  vacuum  and  caused  the  speedy 
destruction  of  the  filament. 

To  surmount  this  difficulty  the  following  plan  was  adopted, 
first,  I believe,  by  Swan,  and  is  now  used  by  all  makers  of 
incandescent  lamps.  While  the  lamp  is  still  attached 
to  the  pump  a current  of  electricity  is  sent  through 
the  filament,  sufficient  to  raise  it  to  a somewhat  higher 
degree  of  incandescence  than  will  be  used  in  actual 
work.  All  the  gas  driven  out  of  the  carbon  is  at  once 
removed  by  the  pump,  and  the  lamp  is  sealed  while  the 
current  is  still  passing. 


Plate  IV. — tiie  swan  lame — old  pattern. 


Swan's  Incandescent  Lamp . 63 

The  Swan  Lamp.  (Plates  TV.  and  VI.) 

The  first  incandescent  lamps  exhibited  in  England  were, 
as  we  said  before,  those  shown  by  Mr.  Swan  to  the  Society 
of  Telegraph  Engineers  on  November  24,  1880.  The  form 
of  lamp  then  exhibited  is  shown  in  Plate  IV.  It  has  been 
greatly  improved  upon  since,  but  the  old  form  is  of  so 
great  an  historical  interest  that  I have  given  a somewhat 
full  description  of  it. 

Plate  IV.  shows  an  old  Swan  lamp  of  25-candle  power,  full 
size.  Fig.  27  shows  some  of  the  details  of  it  on  an 
enlarged  scale. 

The  Filament. 

The  filament  consists  * of  a flat  plait  or  round 
thread  of  cotton,  which  is  parchmentised  by 
immersing  it  in  a mixture  of  two  parts  of  sul- 
phuric acid  to  one  of  water.  The  thread  is  left 
in  the  acid  just  long  enough  to  effect  the 
required  change,  and  then  removed,  and  quickly 
and  thoroughly  washed  in  water,  so  as  to  remove 
the  last  trace  of  acid. 

This  operation  has  the  effect  of  completely 
destroying  the  fibrous  character  of  the  cotton, 
so  much  so  that  the  parchmentised  thread  after 
drying  becomes  smooth  and  transparent  like 
silkworm  gut.  Before  it  is  carbonized,  the 
parchmentised  thread  is  passed  through  dies, 
which  reduce  it  to  a uniform  cross  section.  It 
is  then  wound  on  rods  of  carbon  or  earthen- 
ware, so  as  to  give  the  required  form  to  the 
filament  preparatory  to  carbonization.  The 
most  usual  form  so  imparted  to  the  filament  in 
this  manner  is  the  loop  and  spiral  represented 
in  Plate  V. 

The  ends  of  the  filament  are  thickened,  by 
having  either  more  thread  or  some  bibulous 

* Swan’s  Specification,  No.  4933,  Nov.  27,  1880. — All  specifications  are 
published  by  the  Commissioners  of  Patents,  Sale  Department,  38,  Cur- 
sitor  Street,  Chancery  Lane,  E.C. 


Fig.  27. 


64 


Electi'ic  Lighting . 

paper  wound  round  them,  either  before  or  after  the  thread 
is  carbonized.  The  thickened  part  is  carbonized  in  the 
same  manner  as  the  filament  itself. 

Carbonization  of  the  delicate  filaments  wound  round  the 
rods  as  described  is  effected  by  burying  them  in  a mass  of 
powdered  charcoal  contained  in  a crucible,  and  then 
raising  to  a very  high  temperature  in  a furnace  during 
several  hours. 

In  mounting  the  filaments,  the  thickened  ends  are  inserted 
into  split  metal  tubes,  which  are  made  to  clip  them  tightly 
by  means  of  a sliding  ring.  The  two  tubes  are  continued 
as  half- tubes,  as  seen  in  fig.  27  and  in  Plate  II.,  and  form 
the  conductors  by  which  the  current  is  conveyed  to  the 
carbon.  They  are  supported  by  being  tied  to  a glass  rod 
which  forms  part  of  the  neck  of  the  lamp.  Platinum  wires 
are  attached  to  the  upper  ends  of  the  metal  tubes,  and 
pass  out  through  the  glass,  being  melted  into  it.  In  order 
to  prevent  any  leakage,  little  platinum  caps  were  fixed  to 
the  wire  just  where  it  comes  through  the  glass,  and 
melted  on  to  the  glass  outside. 

The  reason  of  having  so  long  a neck  to  the  lamp,  was  that 
it  was  thought  if  it  were  shorter  sufficient  heat  might  be 
conducted  from  the  carbon  to  the  platinum  wire  and  cap 
to  risk  cracking  the  glass. 


The  Exhaustion. 

In  order  to  exhaust  the  air,  a tube,  with  a narrow  neck 
or  contraction  in  it,  is  left  attached  to  the  globe.  This 
is  connected  to  a Sprengel  or  other  suitable  air-pump. 
After  the  lamps  have  been  under  exhaustion  for  about 
half  an  hour,  the  vacuum  is  tested  by  an  induction 
coil,  and  if  found  to  be  non-conducting,  the  blow-pipe  is 
again  applied  at  the  point  of  junction  of  the  lamp  with  the 
exhaust  tubes,  an  d the  two  are  gently  drawn  asunder.  This 
is  so  managed  that  a very  short  spike  is  left  at  the  point  of 
severance.  This  spike  forms  the  little  point  seen  at  the 
bottom  of  Plate  IV. 


*<.I  U 1 49  ft  ft  31  I 


WESTERN  ELECTRIC  COMPA.W 


Manufacture  of  Swan  Lamps. 


65 


The  Air-Pump  (Stearn’s  Improved  Sprengel). 

(Plate  Y.) 

Tlie  ordinary  Sprengel  pump  consists  of  glass  tubes 
down  which  mercury  flows  in  a broken  stream  or  in  drops. 
Near  the  top  of  the  tubes  are  side  openings  connected  to 
the  chamber  to  be  exhausted.  Air  enters  from  this  chamber, 
and  becoming  compressed  between  consecutive  mercury 
drops,  is  carried  away,  and  the  process  is  repeated  until  the 
chamber  is  completely  exhausted. 

In  the  ordinary  form  of  the  Sprengel  pump  the  action  is 
very  slow,  particularly  in  the  later  stages,  and  as  the 
mercury  at  the  bottom  end  of  the  tube  is  exposed  to  the 
atmospheric  pressure,  the  tubes  have  to  be  over  thirty  inches 
high,  in  order  that  the  weight  of  the  column  may  overcome 
that  pressure. 

In  Mr.  Stearn’s  improved  form  of  the  pump  (Plate  V.), 
used  in  the  manufacture  of  Swan  lamps,  the  mercury  is 
automatically  raised  to  the  overflow  level  by  atmospheric 
pressure,  the  atmospheric  pressure  upon  the  outgoing 
mercury  being  decreased  by  connecting  the  outflow  tube  to 
a vacuum  produced  by  an  ordinary  mechanical  air-pump. 

Plate  Y.  shows  Mr.  Stearn’s  pump  in  detail  \ its  action 
is  as  follows  : — 

Mercury  is  put  into  the  outer  tube  on  the  right  till  it 
nearly  fills  the  bulb  T.  It  is  prevented  from  rising  on  the 
left  hand  tube  by  the  valve  v.  The  inner  tube  t passes 
through  an  air-tight  stopper  at  s. 

The  tap  B is  first  opened  so  as  to  remove  the  greater 
portion  of  the  air  from  the  bulbs  T,  U,  r,  and  F. 

B is  then  closed  and  A opened.  This  causes  the  mercury 
to  spout  up  through  the  tube  t into  the  bulb  U,  whence  it 
flows  through  the  vessel  r and  out  of  the  jets  j,  and 
through  the  fall  tubes  /,  carrying  with  it  the  residue  of 
the  air  and  also  the  air  from  the  lamp-tube  l and  from  the 
lamps  LL.  The  air  thus  sent  into  the  reservoir  F is 
drawn  thence  through  B to  the  vacuum  chamber  by  the 
mechanical  pump.  By  the  action  of  an  automatically 
worked  two-way  cock,  A is  periodically  and  alternately 

F 


66 


Electric  Lighting . 

connected  to  the  atmosphere  and  to  a vacuum  chamber 
in  which  the  vacuum  is  maintained  by  the  action  of  a 
mechanical  pump. 

This  causes  the  valve  v to  alternately  open  and  close, 
and,  consequently  on  this  periodical  action,  the  mercury  flows 
through  the  overflow  tube,  and  in  this  way  a continual 
supply  of  mercury  is  given  to  the  fall  tubes. 

d is  a drying  tube  to  remove  moisture.  The  trough  at 
the  bottom  of  the  pump  is  to  catch  the  mercury  in  case  of 
a tube  breaking. 


Mounting. 

For  convenience  in  attaching  the  lamp  to  the  wires  bringing 
the  current  to  it,  the  stem  of  the  old  lamp  (Plate  IV.)  was 
enclosed  in  a cardboard  tube  about  J inch  thick.  Brass 
springs  attached  to  the  two  sides  of  this  were  connected 
respectively  to  the  two  platinum  caps,  by  means  of  the  little 
screws  seen  at  the  top  of  Plate  IV. 

At  the  end  of  the  wall-bracket  or  other  stand  which 
carried  the  lamp  was  fixed  a wooden  tube,  into  which  the  paper 
tube  on  the  lamp  just  fitted.  Inside  this  wooden  tube  were  two 
metal  plates,  to  which  were  attached  the  wires  supplying  the 
current. 

When  the  lamp  was  put  into  position,  by  having  its  stem 
slid  into  the  tube,  the  springs  pressed  on  the  metal  plates, 
and  at  once  made  the  connection. 

We  see  that  in  case  of  a lamp  breaking  down,  it  could  be 
removed  and  a new  one  substituted  by  any  servant,  and 
without  the  necessity  of  employing  a person  who  under- 
stands electrical  connections.  The  lamp  can  of  course  be 
used  either  side  up,  or  horizontally,  or  in  any  other  position 
that  is  preferred. 

Efficiency. 

The  20-candle  lamps  of  the  old  pattern  had  each  a re- 
sistance of  from  45  to  150  ohms  when  cold ; or  25  to  75  ohms 
when  hot.  They  required  from  1 to  \\  amperes  of  current, 
and  30  to  50  volts  E.M.F. 


Plate  VI. — the  swan  lamp— new  pattern. 


The  New  Swan  Lamp . 


67 


To  determine  the  horse-power  absorbed  in  a lamp  we 
make  use  of  equation  (7),  page  15, — 


H.P.  = 


E C 
746' 


For  example  : What  horse-power  is  expended  in  a lamp 
requiring  45  volts  and  1*5  ampere? 

We  have  from  (7), — 


HP-  45  1‘5 
' * 746 


•091, 


or  a little  less  than  -^L-  H.P. 


Current,  E.M.F.,  and  Copper. 

The  horse-power  expended  in  a lamp  depends  on  the 
product  of  the  current  into  the  electromotive  force  at 
wThich  it  is  supplied,  and  is  therefore  the  same  as  long  as 
the  product  is  constant,  whether  the  E.M.F.  is  small  and 
the  current  large,  as  in  the  old  pattern,  or  vice  versa.  We 
note  that  the  higher  the  resistance  of  the  filament,  i.e. 
the  longer  and  thinner  it  is,  the  more  E.M.F.  is  required 
to  drive  the  current  through  it,  and  the  less  current  is 
required  to  produce  a given  quantity  of  energy  in  the  form 
of  heat  and  light  in  the  lamp. 

The  quantity  of  copper  required  for  a conductor  of  given 
length  depends  only  on  the  current  it  has  to  carry,  and  not 
on  its  E.M.F.  We  thus  see  that  every  improvement  in  the 
lamps  which  enables  the  filament  to  be  made  thinner  and  longer, 
and  so  diminishes  the  current  used , proportionably  diminishes 
the  quantity  of  copper  in  the  mains.  As  this  copper  is  one 
of  the  most  expensive  items  in  an  electric  light  plant, 
improvement  in  this  direction  is  extremely  important. 

The  New  Swan  Lamp,  1883  Pattern.  (Plate  VI.) 

Since  the  introduction  of  the  original  Swan  lamp,  several 
modifications  have  been  made  in  its  form  and  other  details. 
Plate  VI.  represents  the  latest  form,  in  which  the  filament  is 
much  longer  and  thinner  than  in  the  old  pattern,  being 
about  five  inches  long  and  ,005  inch  in  diameter.  It 


68 


Electric  Lighting . 

requires  an  E.M.F.  of  100  to  120  volts  to  bring  it  to  normal 
incandescence  of  20  candle-power. 


Process  op  Manufacture. 

Figs.  28  to  33  show  the  manner  in  which  the  several  parts 
of  the  new  Swan  lamp  are  put  together. 

The  filament  is  attached  to  its  platinum  wires 
and  mounted  on  a glass  bridge,  as  in  fig.  28,  little 
beads  of  glass  being  also  formed  on  the  wires 
where  they  are  to  pass  through  the  walls  of  the 
lamp. 

Fig.  28.  The  globe  is  blown  as  in  fig.  29,  and  with  a 
sharp  file  is  cut  into  two  pieces,  as  in  fig.  30. 


Fig.  30. 


The  carbon  and  platinum  wires  are  inserted,  and  the  latter 
fused  on  by  the  blow-pipe,  as  in  fig.  31. 


Fig.  31. 


Manufacture  of  the  New  Swan  Lamp.  69 

The  two  portions  of  the  globe  are  joined  again  by 
the  blow-pipe,  as  in  fig.  32,  and  the  lamp  is  completed 


in  the  form  shown  in  fig.  33,  and  is  ready  to  attach  to  the 
pump. 

The  following  are  the  results  of  some  experiments  in  the 
efficiency  of  the  new  Swan  lamp  : — 


70 


Electric  Lighting. 


j Candle  - 
| Power  of 
Lamp. 

Current 

in 

Amperes. 

E.M.F. 

in 

Volts. 

Resist. 

in 

Ohms. 

(Hot.) 

| Volt- 
amperes 
per 

| Candle. 

| 

I 

Candles  j Lamps 
perH.P.  ! perH.P. 

16 

| 

•62 

98 

158 

3-74 

199 

12 

! 16 

*68 

97-1 

154 

3-69 

202 

12 

1 16 

•726 

78 

107 

35 

213 

12 

18 

•63 

100 

158-7 

35 

213 

12 

! 18 

•64 

99-1 

154-8 

352 

212 

12 

18 

•75 

80 

107  j 

3-3 

226 

12 

20 

•65 

102 

157 

3*32 

225 

11 

20 

•66 

100 

151-5 

33 

226 

11 

20 

•76 

82  | 108  | 

3-1 

240 

12 

It  will  be  noticed  that  in  tbe  new  types  of  Swan  lamp  the 
connection  between  tbe  platinum  wires  and  tbe  carbon  fila- 
ment is  altered  from  tbe  original  type.  The  carbon  filament 
is  in  tbe  new  types  connected  directly  with  the  platinum 
wires  which  pass  through  the  stem  of  the  glass  bulb. 

The  Holder. 

A great  number  of  holders  of  different  forms  are  in  use. 
The  one  which  I have  adopted,  after  experience  of  a great 
many,  is  shown  in  Plate  VI.,  about  full  size.  It  consists  of 
a block  of  box- wood,  to  which  is  fixed  a spring  wire  with  a 
ring  which  goes  round  the  neck  of  the  lamp,  and  two  springs, 
one  of  which  hooks  into  each  of  the  platinum  rings  of  the 
lamp.  These  contact  springs  are  attached  to  little  brass 
binding  screws  at  the  sides  of  the  wood. 

It  is  important  that  both  the  contact  wires  should  be 
springs,  as  if,  as  was  the  case  in  some  of  the  earlier  holders, 
they  are  made  simple  hooks,  then  the  pressure  of  the  spring 
which  holds  the  neck  of  the  lamp  secures  a good  contact 
with  one  terminal  only,  and  any  vibration  breaks  contact  at 
the  other. 


Swan's  Incandescent  Layups. 


7i 


Special  Lamps. 

Figs.  34  and  35  represent  miniature  lamps  mounted  for 
surgical  purposes.  In  these  lamps  water  circulates 
in  the  space  between  the  lamp  itself  and  an  outer 
glass  tube,  to  keep  the  lamp  cool  enough  to  permit 
of  its  introduction  into  the  internal  parts  of  the 
living  body.  Miniature  lamps  have  been  set  in 
brooches  and  shirt  studs  and  also  made  to  form  the 
petal  of  an  artificial  flower.  Fig.  34. 

Lamps  somewhat  larger  than  these,  of  about  2j> 
candle-power,  in  connection  with  small  portable 
accumulators  of  two  or  three  cells,  have  been  used 
for  stage  effects.  _ 

Swan  lamps  have  been  made  with  the  carbon 
filament  so  short  that  two  volts  have  sufficed  for 

Fig.  35.  rendering  them  incandescent,  and  which  can  there- 
fore be  used  with  one  or  two  cells  of  a battery.  Mr.  Stearn, 
the  coadjutor  of  Mr.  Swan,  has  applied  such  lamps  to  the 
microscope. 

Fig.  36  represents  a microscope  lamp  mounted  on  a 
stand. 


72 


Electric  Lighting . 


Swan’s  Miner’s  Lamp. 

Figs.  37  and  38  illustrate  a lamp  arranged  by  Mr.  Swan 
for  use  in  fiery  mines.  The  ordinary  lamp  is  enclosed  in 
a massive  globe  of  very  thick  glass,  surrounded  by  wire 
guards.  In  case  of  the  lamp  itself  being  broken,  the  air 


in  the  outer  globe  would  suffice  for  the  combustion  of 
the  carbon  filament,  and  would  at  once  extinguish  the  light. 
The  only  danger  attending  the  use  of  this  lamp  is,  that  if 
the  wire  bringing  the  current  to  it  were  to  be  accidentally 


The  Edison  Lamp . 


73 


broken,  a spark  would  occur  between  the  broken  ends  and 
might  fire  the  mine ; or  a spark  might  be  produced  if  the 
naked  wires  were  exposed,  and  metallic  connection  acci- 
dentally made  and  broken  between  them — as,  for  instance, 
by  a man  dropping  a pick  or  drill  upon  them,  and  picking 
it  up  again.  To  guard  against  these  dangers  the  wires 
are  made  very  strong,  and  are  very  thickly  covered  with 
insulating  material. 

The  lamp  is  also  used  by  divers  in  submarine  work. 

The  Edison  Lamp. 

The  Edison  lamp  is  shown  in  fig.  39.  The  carbon 
filament  is  generally  prepared  from  a strip  of  bamboo,  which 
is  cut  to  the  right  shape,  and  has  thickened  ends  left  on  it. 
Sometimes  it  is  made  from  paper  or  cardboard. 

Whatever  substance  is  used  is  carbonized  by  being  placed 
in  a crucible  surrounded  by  powdered  charcoal,  and  raised 
to  a high  temperature  in  a furnace. 

There  is  an  essential  difference  between  the  Swan 
filament  and  the  Edison.  In  the  Edison  filament  the 
cellular  structure  of  the  fibrous  material  is  preserved. 
In  the  Swan  it  is  effaced  by  the  process  of  parchmentization. 

The  thickened  ends  being  cemented  on  to 
the  platinum  wire,  the  joints  are  electro- 
plated with  copper  to  ensure  good  contact. 

The  lamp  is  exhausted  in  much  the  same 
manner  as  the  Swan  lamp. 

The  method  of  connecting  the  lamp  to  the 
line  wires  is  as  follows.  The  screw,  and 
the  conical  collar  seen  just  above  it  in  fig. 

39,  are  insulated  from  each  other,  and  are 
connected  respectively  to  the  two  ends  of  the 
filament.  The  screw  socket  in  the  wall 
bracket  is  connected  to  one  of  the  line 
wires,  and  a hollow  metal  cone  just  above  it 
is  connected  to  the  other.  On  the  lamp 
being  screwed  into  its  socket,  one  contact  is  completed 
through  the  screw,  and  the  other  through  the  cones,  which 
press  one  into  the  other  as  the  lamp  is  screwed  home. 


Fig.  39. 


74 


Electric  Lighting . 

The  lamps  are  at  present  made  in  two  sizes  of  8-candle 
and  16-candle  power  respectively. 

The  following  are  details  of  experiments  on  six  of  the 
16-candle  Edison  lamps  : — 


No.  on 
Lamp. 

Candle- 

Power. 

Resistance. 
Cold.  | Hot. 

Volts  at 
Lamp. 

Current. 

Amperes. 

Energy. 

HP. 

Expended. 

Candles  per 
H.P.,  or 
Efficiency. 

_s| 

fi 

8 

105*3 

58-8 

51 

•868 

•0593 

135 

11 

8 

97-7 

57 

51 

•896 

•0612 

131 

£ ' 

U 

8 

95-8 

58-4 

515 

•882 

•0609 

131 

. "S  1 

f4 

16 

257-6 

144-2 

105 

•728 

•1024 

156 

a>© 

a dj 
o gc 

is 

16 

272-7 

150-7 

105’5 

•700 

•0989 

162 

s| 

Is 

16 

242-5 

141-5 

105 

•742 

•1044 

154 

This  gives  an  average  of  145  candles  per  H.P.  expended 
in  the  lamp. 

Efficiency  and  Durability. 

In  comparing  the  amount  of  light  per  horse-power  given 
by  different  incandescent  lamps,  we  must  remember  that  we 
can  increase  it  up  to  almost  any  amount  we  please  by 
working  the  lamp  at  a higher  temperature,  only  by  so  doing 
we  reduce  the  life  of  the  lamp  from  six  months  to  perhaps 
three  months,  or  a few  weeks,  days,  hours,  or  minutes.  Only 
experience  can  show  us  what  is  the  most  economical 
temperature  to  work  at,  having  regard  both  to  the  cost  and 
trouble  of  renewing  the  lamps,  and  to  the  cost  of  the  electric 
current  which  works  them.  This,  “ the  temperature  of 
maximum  economy,”  will  vary  with  the  price  of  coal,  being 
highest  in  places  where  coal  is  dearest,  and  vice  versa . It  will 
be  lowest  of  all  where  water  power  is  used  instead  of  steam  to 
drive  the  dynamos. 

Temperature  Scale  for  Incandescent  Lamps. 

The  efficieccy  of  a lamp  depends  on  the  temperature  to 
which  it  can  be  worked.  We  have,  however,  no  means  of 
measuring  these  high  temperatures  on  the  ordinary  scales ; 
nor,  if  we  had,  would  it  be  of  much  use  to  us. 

I have  therefore  suggested  * that  the  temperature  of  the 
filament  of  an  incandescent  lamp  could  be  measured  by  the 
ratio  of  the  horse-power  expended  on  it,  to  its  surface;  f and 
* Electrician,  Jan.  14,  1882.  f See  Eq.  (30),  page  79. 


75 


The  Maxim  Lamp. 

that  companies,  if  they  were  to  give  guarantees  of  durability, 
might  write  them  in  the  form  : — “ These  lamps  will  last  for 
six  months,  at  a temperature  not  exceeding  such-and-such 
a horse-power  per  square  inch  of  surface.'” 

A lamp  with  an  efficiency  of  150  candles  per  H.P.  would 
use  about  1 H.P.  per  square  inch  of  surface. 

Example. 

A 15-candle  Lane-Fox  lamp  has  a surface  of  y1^-  square 
inch,  a resistance  of  45  ohms  hot,  and  takes  a current 
of  1*3  ampere.  From  equation  (4)  we  have, — 

H.P.  = 13  XP  - 45  = 1 almost  exactly. 

746  10  J 

Thus  10  lamps  give  150  candles,  have  1 square  inclHsur- 
face,  and  take  1 H.P.  to  work  them. 


Fig.  40. 


The  Maxim  Lamp. 

Fig.  40  shows  the  Maxim  lamp  mounted  on  a wall-bracket. 


76 


Electric  Lighting. 

Figs.  41,  42  show  the  method  of  attaching  the  carbon  to 
its  supports.  The  ends  of  the  carbon  are  flattened,  and  are 
held  by  nuts.  As  it  is  difficult  to  obtain  a good  contact 
between  platinum  and  the  hard  carbon  of  which  the  lamp 
filaments  are  made,  Mr.  Maxim  introduces  washers  of  soft 
carbon  between  the  filament  and  the  nut  and  bolt. 


Fig.  41. 


[U1 

T 


Fig.  42. 


In  the  Maxim  system  the  filament  is  carbonized  by  heat- 
ing in  the  usual  way,  but  during  the  whole  process  it  is  kept 
surrounded  by  an  atmosphere  of  gasoline  or  other  gas  rich  in 
carbon ; and  when  the  lamp  is  completed  and  has  been  ex- 
hausted, gasoline  vapour  is  let  in  and  pumped  out  again. 
When  this  has  been  repeated  two  or  three  times  every  trace 
of  oxygen  is  removed  from  the  globe,  and  the  residual  gas  in 
the  vacuum  is  pure  gasoline. 

Fig.  43  shows  the  arrangement  used  for  carbonizing. 


Fig.  43. 


There  is  a flat  crucible,  in  which  the  cardboard  strips,  pre- 
viously cut  to  the  right  shape,  are  placed.  They  are  laid 
between  sheets  of  cardboard,  and  these  again  between  metal 
plates.  The  metal  plates  are  somewhat  smaller  than  the 
crucible,  so  as  to  allow  the  carbon  gas  to  circulate  round 
them.  The  layers  of  metal  and  cardboard  about  two-thirds 
fill  the  crucible,  which  is  then  filled  up  with  sand. 

The  gas  is  supplied  by  causing  ordinary  coal  gas  to 


Manufacture  of  the  Maxim  Lamp.  77 

bubble  through  a bottle,  containing  gasoline  or  other 
volatile  hydrocarbon  oil. 

“ The  carbonizer  thus  filled  is  placed  over  a gas  flame  or 
upon  a stove,  and  heated  to  a temperature  sufficiently  high 
to  expel  the  aqueous  vapour  contained  in  the  pores  of  the 
material,  but  not  sufficiently  high  to  char  the  material  to 
any  considerable  extent,  and  the  carburetted  gas  is  admitted 
to  the  carbonizer  through  the  pipe  a,  fig.  43,  and  ignited,  where 
it  escapes  through  the  sand  at  the  top.  After  the  material 
has  been  subjected  to  this  heating  for  a considerable  time, 
say  ten  or  twelve  hours,  the  carbonizer  is  placed  in  a muffle- 
furnace  and  raised  to  a white  heat,  and  kept  there  until  all 
the  material  is  thoroughly  charred,  the  gas  being  all  the 
time  supplied  to  the  carbonizer  through  the  pipe,  and  cir- 
culating about  the  forms  so  as  to  envelope  them  on  all  sides. 

“ The  function  of  the  gas  during  the  first  part  of  the 
process  is  to  all  appearance  to  permeate  the  pores  of  the 
material  and  drive  out  the  aqueous  vapour  and  air  contained 
in  them  as  far  as  possible  : and  its  function  during  the 
latter  or  charring  part  of  the  process  seems  to  be  to  pro- 
tect and  consolidate  the  carbon  of  the  material.  When  the 
hydrogen  and  other  constituents  are  dissociated  from  the 
carbon  of  the  material  by  the  heat  of  the  furnace,  the 
surrounding  hydrocarbon  vapour  or  gas  is  also  apparently 
decomposed;  and  some  part  of  the  carbon  thus  liberated, 
especially  that  which  is  contained  in  the  pores  of  the  material, 
is  apparently  deposited  upon  the  carbon  of  the  forms,  and 
serves  to  consolidate  it.  The  hydrogen  when  liberated  does 
not  corrode  the  carbon  of  the  forms ; but  if  it  has  any  ten- 
dency to  again  take  up  carbon,  probably  unites  with  some 
part  of  the  free  carbon  liberated  from  the  gas  or  vapour.”* 
When  the  lamp  is  completed,  the  first  effect  of  the 
passage  of  the  current  is  to  decompose  the  trace  of  vapour 
left  in  the  globe,  and  deposit  the  carbon  from  it  on  the 
heated  filament.  The  inventor  considers  that  “ an  almost 
absolute  vacuum  is  thus  established  in  the  globe.” 

As  the  hottest  points  in  the  filament  are  the  points  where 
it  is  thinnest,  and  therefore  weakest,  more  carbon  will  be 

* Maxim’s  Specification,  No.  1649,  April  21st,  1880. 


78 


Elect nc  L ighting. 

deposited  on  these  points,  and  therefore  the  action  tends  to 
correct  any  unevenness  in  the  carbon. 

It  has  been  frequently  stated  that  Maxim's  lamp  can  be 
run  at  a higher  temperature — that  is,  can  give  more  light 
per  horse-power — than  either  Edison’s  or  Swan's.  This  may 
possibly  be  the  case,  but  I am  not  aware  of  any  experiments 
confirming  this  view.  It  is  true  that  Professor  Ayrton  * 
has  published  an  account  of  experiments  where  these  lamps 
were  run  at  enormous  temperatures,  and  had  a corre- 
spondingly high  efficiency.  The  experiments,  however,  only 
lasted  a few  minutes,  and  the  temperatures  were  generally 
increased  until  the  lamp  broke.  The  figures  obtained  from 
them  therefore  give  no  information  as  to  what  would  be 
the  efficiency  of  the  lamp  at  its  normal  temperature — or,  in 
other  words,  at  what  temperature  a company  would  work 
the  lamp,  if  they  were  giving  a guarantee  that  it  should  last 
six  months. 

The  Lane-Fox  Lamp. 

Mr.  Lane- Fox's  process  is  probably  not  very  different 
from  that  adopted  by  other  manufacturers.  As  it  happens, 
however,  that  I have  had  an  opportunity  of  inspecting  it 
somewhat  closely,  I will  describe  it  in  detail,  not  saying 
that  it  is  better  or  worse  than  other  processes,  but  as  an 
illustration  of  what  the  general  nature  of  all  the  processes  is. 

The  filaments  are  usually  prepared  from  the  fibres  of  the 
bass  broom. 

About  100  pieces  of  fibre,  cut  to  the  right  length,  are  bent 

round  a block  of  carbon,  and 
secured  to  it  by  winding  string 
round,  as  shown  in  fig.  44. 
Some  fifty  of  these  blocks  are 
prepared,  and  placed  in  layers 
one  above  another  in  a crucible, 
all  the  interstices  between  them 
being  filled  with  powdered 
charcoal,  with  which  also  the  crucible  is  filled  up  to  the 
top. 


Engineer , November  25,  1881. 


Plate  VII.— furnace  used  in  manufacture  of  lane-fox  lamps, 


79 


The  Lane- Fox  Lamp . 

The  crucible  is  then  placed  in  a small  furnace  (Plate  VII.), 
and  the  temperature  gradually  raised  up  to  full  white- 
ness. The  crucible,  with  its  contents, 
is  kept  at  a full  white  heat  for  some 
twenty  minutes,  after  which  it  is 
allowed  to  cool.  On  being  taken  out, 
the  fibres  are  found  to  be  converted 
into  hard  carbon  of  a rough,  porous 
texture  and  of  high  and  unequal 
resistances. 

Plate  VII.  shows  the  details  of  the 
arrangements  of  the  furnace  used  by 
Mr.Lane-Fox  as  constructed  by  Messrs. 

Fletcher. 

We  note  that  there  is  a perforated 
diaphragm  which  causes  the  draught 
to  circulate  evenly  round  the  crucible. 

A furnace  and  crucible  of  the  size 
shown  will  carbonize  about  5000  fila- 
ments at  one  heating. 

The  next  process  is  to  gauge  the  fibres  with  a wire  gauge 
reading  to  inch  (fig.  45) ; they  are  then  sorted,  so  that 
all  of  the  same  diameter  are  placed  together  for  the  making 
of  lamps  of  the  same  radiating  surface. 

If  S be  the  surface,  L the  length,  and  D the  diameter,  we 
have, — 

S = 3-1416  D L (44) 

Example. 

A filament  is  2 inches  long  and  '018  inch  diameter,  what 
is  its  surface  ? 

We  have  from  (44), — 

S = 3'1416  x ’018  x 2 = '1031  of  a square  inch. 

The  next  operation  is  the  equalizing  of  the  resistances, 
and  the  hardening  and  smoothing  of  the  carbon.  This 
is  accomplished  by  means  of  what  is  called  a flashing- 
bottle. 

The  carbons  are  placed,  one  by  one,  with  their  two  ends 


8o 


Electric  Lighting. 

in  two  spring  clips,  so  that  a current  can  be  sent  through 
them.  These  clips  are  fixed  into  a cork,  so  that  they  can 
be  placed  inside  a bottle,  through  which  a stream  of 
coal-gas  is  constantly  flowing. 

On  a current  being  sent  through  the  filament  so  as  to 
render  it  incandescent,  carbon  from  the  gas  commences  to 
deposit  on  it,  and  the  filament  becomes  denser  and  smoother, 
and  its  resistance  rapidly  diminishes.  The  process  is 
stopped  when  the  resistance  has  reached  exactly  the  desired 
amount.  As  the  resistance  diminishes,  the  brightness  of 
the  light  given  by  the  filament  in  the  flashing-bottle 
increases. 

When  the  workman,  judging  by  the  light,  considers  that 
the  resistance  has  reached  its  proper  value,  he  removes  the 
filament  from  the  bottle  and  tests  its  resistance  (cold)  by  a 
Wheatstone’s  bridge.  After  a little  practice  the  workmen 
are  able  to  obtain  the  resistance  correct  to  about  one  ohm, 
by  judging  the  brightness  of  the  light,  in  one  or  two  tries, 
or  “ flashes/'  for  each  filament. 

By  means  of  the  ohmmeter,*  it  might  be  possible  to  obtain 
the  hot  resistance  accurately  without  taking  the  filament  out 
of  the  bottle. 

The  thickened  ends  of  the  filaments  consist  of  two 
little  cylinders  of  carbon,  which  are  drilled  from  end  to 
end.  The  ends  of  the  filament  are  inserted  into  the  holes 
at  one  end  of  each  cylinder,  and  the  platinum  wires 
which  pass  through  the  glass  of  the  lamp  are  inserted  into 
the  other  ends.  The  filament  and  platinum  wires  are 
secured  by  a cement  of  which  Indian  ink  is  the  chief 
ingredient. 

The  construction  of  the  glass  part  of  the  lamp  is  seen  in 
Plate  VIII.  and  in  figs.  46  and  47. 

The  globe  is  prepared  with  a sufficiently  wide  neck  to 
admit  the  carbon  loop  (which  is  sufficiently  elastic  to  be 
considerably  bent  without  breaking). 

A glass  piece  (fig.  46)  is  prepared,  having  two  narrow 
tubes,  into  the  points  of  which  are  melted  little  pieces 
of  platinum  wire,  about  T3^  inch  long.  The  carbon  is 

* Page  56. 


Jr'LAIE  MIL — THE  LANE- VOX  LAAIt', 


tiwiNttiiintf  ytr'fliiiMtHl  LlSnAliT 


-WESTERN  IIWMMWM 

I he  Lane- pox  Lamp. 


81 


attached  to  these  wires  in  the  manner  already  described, 
and  the  forked  piece  being  inserted  in  the  globe,  the 
neck  of  the  globe  is  melted  on  to  the  wide  part  of  it 
in  a blow-pipe  flame.  The  upper  piece  is  taken  off, 
so  that  the  neck  is  hollow  and  funnel-shaped,  as  shown 
in  fig.  47. 


Fig.  47. 


A fine  tube  is  then  melted  into  the  side  of  the  neck 
for  exhaustion.  A little  mercury  * is  put  from  outside  into 
each  of  the  tubes  of  the  forked  piece,  and  copper  wires  put 
in  so  as  to  dip  into  the  mercury  at  C.  The  mercury  forms 
an  electrical  connection  between  the  copper  and  platinum, 
and  at  the  same  time  prevents  any  leakage  ot  air  where 
the  platinum  passes  through  the  glass.  The  hollow  neck 

* The  whole  of  this  part  of  the  process  is  antiquated,  but  it  is  of  some 
historical  value  as  showing  the  difficulties  which  had  to  be  overcome  in 
the  early  days  of  lamp  manufacture. 


G 


82 


Electric  Lighting. 

is  filled  up  with,  cotton- wool  (B),  and  with  plaster  of 
Paris  (A),  which  holds  the  copper  wires  and  the  mercury 
in  their  places.  The  lamp  is  now  ready  for  exhaustion. 

This  is  accomplished  by  means  of  a pump  invented  by 
Mr.  Lane-Fox,  and  shown  in  Plate  IX. 

The  Lane-Fox  Pump.  (Plate  IX.) 

The  pump  consists  essentially  of  two  large  globes  con- 
nected by  a tube,  partly  of  glass  partly  of  india-rubber.  The 
glass  part  (A)  of  the  tube  is  vertical,  and  is  some  35  inches 
long,  and  one  globe  is  fixed  at  the  top  of  it.  The  other 
globe  is  moveable,  and  is  connected  to  the  lower  end  of  the 
fixed  glass  tube,  by  a length  of  india-rubber  tube  sufficient 
to  allow  the  moveable  globe  to  be  hung  by  its  wire  handle 
to  either  of  two  hooks,  one  of  which  brings  it  above  the  fixed 
globe,  while  the  other  is  below  the  level  of  the  bottom  of  the 
fixed  glass  tube.  The  top  of  the  moveable  globe  is  open  to 
the  air. 

Enough  mercury  is  put  in  the  apparatus  to  fill  the  tube 
and  one  globe. 

A side  tube  branches  out  from  the  fixed  tube  near  the  top, 
and  turns  up  and  passes  vertically  a little  higher  than  the 
top  of  the  fixed  globe. 

A lamp,  or  generally  two  lamps  at  a time,  are  attached  to 
the  top  of  this  tube. 

In  the  vertical  part  of  the  side  tube  is  a valve  consisting  of 
a cylindrical  enlargement,  inside  which  lies  a hollow  glass 
stopper.  On  mercury  rising  in  the  side  tube,  the  stopper 
floats  up  and  closes  the  tube  sufficiently  tight  to  stop  mer- 
cury from  passing  up  to  the  lamps.  The  mercury  surrounding 
the  stopper  prevents  air  passing  it. 

At  the  top  of  the  fixed  globe  is  a valve  worked  by  hand, 
consisting  of  a glass  stopper  surrounded  by  a second  smaller 
globe.  To  close  the  valve  the  stopper  is  inserted,  and  a 
little  mercury  or  sulphuric  acid  put  in  the  smaller  globe  sur- 
rounding it.  The  stopper  prevents  the  liquid  from  passing 
from  the  small  to  the  large  globe,  and  the  liquid  prevents 
the  entrance  of  air. 


83 


Lane-Fox' s Air-pump . 

When  the  pump  is  worked,  the  moveable  globe  is  hung  on 
the  upper  hook,  so  that  the  mercury  rises  in  the  fixed  tubes. 
In  the  side  tube  it  rises  as  far  as  the  valve,  which  it  closes ; 
while  in  the  main  tube  it  rises  and  fills  the  fixed  globe,  the 
stopper  being  withdrawn  to  allow  the  air  to  escape. 

Some  strong,  pure  sulphuric  acid  is  then  put  into  the 
smaller  globe,  and  the  moveable  globe  lowered  sufficiently  to 
allow  a little  of  the  acid  to  enter  the  large  fixed  globe,  where 
it  lies  on  the  top  of  the  mercury.  The  stopper  is  then  in- 
serted, and  the  moveable  globe  completely  lowered  and 
hung  on  the  lower  hook. 

The  mercury  sinks,  leaving  a complete  vacuum  in  the 
upper  globe  until  the  column  is  lower  than  the  opening  of 
the  side  tube,  when  the  valve  in  the  latter  opens  and  the 
air  in  the  lamps  rushes  into  the  large  globe. 

The  moveable  globe  is  then  again  raised,  and  the  rising 
mercury  closes  the  valve  leading  to  the  lamp ; and  the  air, 
being  directed  into  the  fixed  globe,  escapes  through  the 
stopper  valve,  which  is  opened  for  the  purpose.  Meanwhile 
the  sulphuric  acid  removes  every  trace  of  moisture.  We  see 
that  the  ratio  of  the  quantity  of  air  in  the  lamp  after  each 
stroke  is,  to  that  before  the  stroke,  as  the  volume  of  the 
lamp  is  to  the  sum  of  the  volumes  of  the  lamp,  tube,  and 
fixed  globe. 

After  a few  minutes'  work  the  lamp  is  completely  ex- 
hausted, as  is  shown  by  the  sharp  click  with  which  the 
mercury  strikes  the  top  of  the  fixed  globe  on  the  moveable 
globe  being  raised. 

The  filaments  are  then  brought  to  a state  of  vivid  incan- 
descence, and  the  pumping  recommenced,  and  continued  for 
a few  minutes  more,  until  the  greater  portion  of  the  gas  con- 
tained in  them  has  been  expelled  and  removed.  They  are, 
however,  kept  on  the  pump  and  incandescent  for  some  twelve 
hours,  a stroke  being  taken  only  occasionally  to  remove  the 
last  trace  of  gas.  The  lamps  are  then  sealed  off  in  the 
ordinary  way,  and  are  ready  for  use. 


G 2 


84 


Electric  Lighting . 


CHAPTER  VII. 

Arc  Lamps. 

In  arc  lamps,  as  we  have  already  stated,*  the  resistance 
which  converts  the  current  into  heat  is  that  of  the  heated 
air  between  the  ends  of  two  carbon  rods,  from  one  to  the 
other  of  which  the  current  passes.  The  light  is  produced 
by  the  incandescence  of  the  end  of  the  carbon  poles  and  of 
the  minute  particles  of  carbon  which  become  detached,  and 
float  in  the  heated  air  between  them.  The  heated  air  con- 
taining the  particles  of  carbon  forms  what  is  called  the 

electric  arc.” 

The  carbon  rods  vary  in  diameter  from  J inch  in 
the  smallest  lamps  made,  to  3J  inches  in  a lamp  recently 
constructed  by  the  Brush  Company. 

The  carbon  rods  slowly  burn  away,  and  therefore  have  to 
be  continuously  fed  forward  by  suitable  machinery,  so  as  to 
keep  “ the  resistance  of  the  arc  ” as  constant  as  possible. 
On  the  steadiness  of  the  feeding  machinery,  and  on  its 
sensitiveness  to  minute  changes  in  the  resistance,  depend  in 
a great  measure  the  steadiness  and  freedom  from  flickering 
of  the  light. 

It  is  also  necessary  that  all  arc  lamps  should  light  them- 
selves when  the  current  is  started,  i.e.,  that  when  no  current 
is  passing,  the  carbons  should  be  in  contact,  and  that  when 
the  current  commences  to  flow,  they  should  be  instantly 
separated  to  a distance  giving  an  arc  of  the  required  re- 
sistance. This  distance  will  vary  according  to  E.M.F., 
size  of  lamp,  &c.,  from  ^ inch  to  f inch. 

An  immense  number  of  regulators  have  been  constructed 


Arc  Lamps. 


85 


by  different  inventors,  but  they  may  all  be  divided  into 
some  three  or  four  general  types.  I propose  in  the  present 
chapter  to  describe  some  three  or  four  lamps  only,  selecting 
those  which  are  in  actual  commercial  use,  and  which  differ 
as  widely  as  possible  among  themselves. 

The  qualities  required  in  lamps  are  different  according 
to  the  service  for  which  they  are  intended. 

For  lighthouse  work  it  is  absolutely  necessary  that  the 
light  shall  never  be  extinguished  for  an  instant,  and  that 
the  mechanism  shall  be  strong,  and  that  an  ordinary  light- 
keeper  shall  be  able  to  manage  it.  Expense,  weight,  and 
bulk  are  matters  of  no  consideration  whatever;  neither  is 
there  any  objection  to  the  mechanism  being  below  the  arc, 
as  the  light  is  not  required  to  be  directed  vertically  down- 
wards. Slight  pulsations  in  the  light  are  not  a serious 
defect.  The  arc  must  always  be  kept  in  the  focus  of  the 
reflector,  so  both  carbons  must  be  fed  forward. 

In  lamps  intended  for  street  lighting  the  chief  considera- 
tion is  steadiness  and  freedom  from  flickering.  They  must 
be  moderately  cheap,  and  not  too  heavy  to  hang  on  an  ordi- 
nary lamp-post.  The  whole  of  the  mechanism  must  be  above 
the  light,  so  that  shadows  may  not  be  cast  downwards. 

A temporary  extinction  of  the  light,  though  much  to  be 
deprecated,  would  not,  as  in  the  case  of  the  lighthouse 
lamps,  be  likely  to  have  consequences  fatal  to  life,  and 
therefore  strength  of  machinery  need  not  be  studied  to  the 
exclusion  of  all  considerations  of  economy. 

Further,  street  lamps  must  be  so  constructed  that  several 
can  be  worked  in  one  circuit  off  one  machine,  and  so  that 
the  accidental  extinction  of  one  shall  not  affect  the  rest. 
As  a slight  lowering  of  the  position  of  the  light  is  not 
objectionable,  one  carbon  may  be  fixed  and  only  one  fed 
forward. 

Arc  lamps  are  not  suited  for  the  interior  illumina- 
tion of  rooms,  but  when  so  used  considerations  of 
perfect  freedom  from  flickering  outweigh  all  others.  In 
this  case  the  lamp  must  be  also  so  adjusted  as  to  be  free 
from  the  hissing  sound  which  is  often  produced  by  an  elec- 
tric arc. 


86 


Electric  Lighting. 

All  lamps  should  be  constructed  so  that  they  will  burn 
from  dark  to  daylight  of  a winter  night,  say  sixteen  hours, 
without  attention  or  requiring  new  carbons. 

When  lamps  are  worked  by  a direct  current,  the  posi- 
tive carbon  consumes  away  about  twice  as  fast  as  the 
negative ; with  an  alternating  current  the  consumptions  are 
of  course  equal.  The  adjustments  of  springs,  &c.,  required 
when  direct  and  alternating  currents  are  used,  is  somewhat 
different,  as  we  shall  see  later  on. 

The  principal  regulator  lamps  at  present  in  use  are — the 
Serrin  (used  exclusively  for  lighthouses),  and  the  Crompton, 
Brush,  and  Siemens  (used  for  lighting  streets,  stations, 
and  large  buildings). 

The  Jablochkoff  candle  may  be  noted  as  a type  of  a class 
of  arc  lamp  where  the  length  of  the  arc  is  kept  constant, 
but  not  its  resistance. 

The  Serrin  Lamp.  (Plate  X.) 

Plate  X.  is  a drawing  of  a Serrin  lamp. 

The  base  of  the  lamp  contains  clockwork,  which  is 
actuated  by  the  weight  of  the  upper  carbon.  The  racks 
carrying  the  upper  and  lower  carbons  are  connected  by 
cog-wheels,  so  that  as  the  upper  carbon  sinks  the  lower  one 
rises  to  meet  it.  When  the  lamp  is  to  be  used  with  alter- 
nating currents  the  cog-wheels  gearing  into  the  two  racks  are 
of  the  same  size,  and  the  carbons  advance  equally.  When 
direct  currents  are  to  be  used,  the  cog-wheels  are  so  propor- 
tioned that  the  + carbon  moves  twice  as  fast  as  the  — • 
one.  As  the  carbons  move,  the  star-wheel,  which  is  the  last 
wheel  of  the  train,  revolves  very  rapidly,  and  a very  slight 
brake  applied  to  it  is  sufficient  to  lock  the  carbons.  When 
the  carbons  are  in  contact  a current  can  be  sent  through 
the  lamp.  The  current  passes  through  the  electro-magnet, 
which  attracts  its  armature,  and  pulling  a lever  draws  down 
the  lower  carbon-holder,  and,  separating  the  carbons,  forms 
the  arc.  At  the  same  time  the  lever  locks  the  star-wheel, 
and  prevents  the  carbons  from  moving. 

As  the  carbons  burn  away  the  arc  gets  longer,  and  as  its 
resistance  increases,  the  current  in  the  magnet  gets  weaker, 


Pla.te  X. — sekrin’s  arc  lamp, 


Plate  XI.— Crompton’s  arc  lamp— old  pattern. 


The  Serrin  Lamp — The  Crompton  Lamp . 87 

and  the  armature  is  drawn  a little  way  from  it  by  the  spring. 
This  releases  the  star-wheel,  and  the  carbons  approach  each 
other  till  the  current  has  recovered  its  proper  strength, 
when  the  armature  is  - again  attracted  and  the  carbons 
locked.  This  adjustment  is  repeated  at  intervals  until  the 
carbons  are  consumed. 

The  upper  carbon  can  be  brought  exactly  into  line  with 
the  lower  one  by  means  of  the  screws  seen  at  the  top  of 
Plate  X.  Of  the  two  screws  seen  on  the  left-hand  side 
of  the  clock-case,  one  adjusts  the  tension  of  the  spring  and 
so  regulates  the  length  of  the  arc,  the  other  enables  both 
carbons  to  be  raised  or  lowered  together  without  altering 
their  distance  apart,  so  as  to  place  the  arc  exactly  in  the 
focus  of  the  reflector.  The  position  of  the  arc  should  be 
level  with  the  top  of  the  little  bracket  which  is  pivotted  on 
the  tube  inside  which  the  upper  carbon-rack  slides.  The 
bracket  can  be  turned  round  so  as  to  be  close  to  the  arc,  or 
can  be  turned  back  out  of  the  way.  The  lamp  is  simply 
slid  into  position  on  two  brass  rails,  to  which  the  wires  from 
the  machine  are  attached.  In  case  of  any  accident  to  the 
lamp  it  can  be  removed  and  a spare  one  substituted  in  a 
few  seconds,  as  placing  the  lamp  in  position  at  once  makes 
the  connections. 


The  Ckompton  Lamp.  (Plates  XI.  and  XII.) 

Plates  XI.  and  XII.  and  figs.  48,  49,  are  diagrams  of 
three  successive  developments  of  arc  lamps  introduced  by 
Messrs.  R.  E.  Crompton  and  Co.,  of  Chelmsford. 

Plate  XI.  shows  the  first  form,  which  was  invented  by 
Mr.  Crompton  in  1 880,  and  was  an  improvement  on  a still 
earlier  lamp,  which  it  is  unnecessary  now  to  describe.  In 
this  lamp,  which  Mr.  Crompton  calls  the  E pattern,  the 
lower  carbon-holder  has  an  up-and-down  play  of  about 
5-  inch ; it  is  pressed  up  by  the  spring  S,  and  is  pulled 
down  whenever  a current  passes  through  the  magnet  M. 

The  upper  carbon-holder  has  a play  of  about  16  inches. 
When  drawn  up  to  its  highest  position  it  tends  to  sink  slowly 
by  its  own  weight,  actuating  as  it  falls  a train  of  wheel- 


88 


Electric  Lighting. 

work,  which,  causes  the  little  wheel  E to  revolve  with  great 
velocity.  A very  slight  pressure  on  the  rim  of  this  wheel 
will  stop  the  motion  of  the  carbon.  This  pressure  is  gene- 
rally maintained  by  a light  spring,  which  presses  on  the 
wheel  except  when  the  lever  N is  lifted  by  the  passage  of  a 
sufficiently  powerful  current  through  the  magnet  G. 

The  magnet  Gr  is  wound  with  fine  wire,  and  is  of  high 
resistance,  and  is  connected  so  as  to  form  a shunt  to  the 
arc.  The  magnet  M is  wound  with  thick  wire,  and  is  in  the 
main  circuit. 

When  a current  is  sent  through  the  lamp,  the  carbons 
not  yet  being  in  contact,  no  arc  is  formed,  but  the  whole 
current  passes  through  the  shunt  magnet  Gr,  which  lifts  the 
brake- spring,  and  allows  the  upper  carbon  to  descend  until 
it  comes  in  contact  with  the  lower  one. 

The  greater  portion  of  the  current  then  leaves  the  shunt 
magnet,  releasing  the  brake  and  stopping  the  motion  of  the 
upper  carbon,  and  passes  through  the  carbons  and  through 
the  thick -wire  magnet  M.  This  magnet  depresses  the  lower 
carbon-holder  about  \ inch  (the  exact  amount  of  play  to  be 
allowed  having  been  previously  regulated  by  a set-screw), 
and,  separating  the  carbons,  forms  the  arc. 

The  carbons  being  held  in  this  position  commence  to 
burn  away,  until  the  length,  and  therefore  the  resistance  of 
the  arc,  begin  to  exceed  their  proper  limits.  When  this 
occurs  a larger  fraction  of  the  current  passes  through  the 
shunt  magnet  G,  and,  the  brake  being  lifted,  the  upper  car- 
bon sinks.  As  the  upper  carbon  approaches  the  lower  one 
the  resistance  of  the  arc  diminishes,  and  the  current  in  the 
shunt  magnet  G diminishing,  the  brake  again  presses  on  the 
wheel  E,  and  stops  the  motion  of  the  carbon ; and  this 
adjustment  goes  on  continuously  until  the  carbons  are  con- 
sumed. We  note  that  the  adjustment  depends  only  on 
the  relative  resistances  of  the  arc  and  of  the  shunt  magnet, 
and  is  not  affected  by  variations  in  the  total  current.  The 
only  effect  of  a decrease  of  the  current  is  to  cause  the  lamp 
to  burn  with  a shorter  arc. 

The  special  feature  which  distinguishes  Mr.  Crompton^s 
lamp  from  the  numerous  other  lamps  in  which  a train  of 


89 


The  Crompton  Lamp. 

wheelwork  and  a brake  had  previously  been  employed,  is 
the  extreme  lightness  and  delicacy  of  the  brake-spring  and 
its  adjustments,  the  whole  of  the  moving  parts  weighing 
only  a few  grains. 

When  the  brake-lever  is  heavy  the  adjustment  of  the  arc 
does  not  take  place  until  there  has  been  a considerable 
increase  in  its  resistance,  and  then  the  carbon  is  not 
again  stopped  until  the  arc  has  become  too  short ; and  in 
consequence,  the  light  is  constantly  pulsating.  With  the 
light  spring  introduced  by  Mr.  Crompton  the  adjustment 
takes  place  almost  as  the  resistance  begins  to  vary.  In 
fact,  with  a Crompton  lamp  in  good  adjustment  the  wheel 
E is  never  quite  still ; the  varying  pressures  of  the  brake 
simply  appear  to  adjust  the  velocity  of  its  motion.  In 
consequence,  the  Crompton  light  is  extremely  steady. 

The  K pattern  Lamp.  (Fig.  48.) 

Fig.  48  represents  the  lamp,  brought  out  -in  1882,  known 
as  the  K pattern,  and  is  the  joint  invention  of  Messrs. 
Crompton  and  Crabb. 

In  this  lamp  it  will  be  seen  that  the  frame  carrying  the 
wheelwork  is  made  capable  of  motion  in  a vertical  plane 
about  a pivot  fixed  to  the  opposite  guide-rod.  Hanging 
vertically  from  this  frame  and  within  a solenoid,  is  a hollow 
cylindrical  iron  core.  The  solenoid,  which  does  the  work 
of  the  electro-magnets  in  the  former  lamps,  is  differential, 
i.e.  it  is  partly  excited  by  the  current  in  the  main  circuit 
which  passes  through  the  lower  and  thicker  wire,  and  partly 
by  a small  current  circulating  in  the  upper  and  much  thinner 
wire  which  is  connected  as  a shunt  to  the  arc. 

Attached  to  the  upper  or  positive  carbon-holder  is  a cord 
which  passes  over  a pulley  connected  in  the  top  of  the 
frame,  round  another  pulley  connected  to  the  train  of  wheel- 
work,  and  which  corresponds  to  the  toothed  wheel  gearing 
into  the  rack  rod  in  the  older  forms  of  Crompton  lamps ; 
then  down  the  hollow  side  rod,  through  one  half  of  the 
cross-bar  at  the  bottom,  down  the  vertical  tube,  under  a 
pulley  attached  to  the  lower  carbon-holder,  up  the  tube 


90 


Electric  Lighting . 


on  the  other  side,  to  the  other  half  of  the  cross-bar,  where 
it  is  fixed.  The  lower  carbon  moves  up 
and  down  in  the  bottom  tube,  guided  at 
its  lower  end  by  a piston-shaped  socket, 
which  makes  good  spring  contact  with  the 
tube,  and  at  its  upper  end  by  the  nozzle  at 
top  of  the  tube,  and  is  supported  by  its 
pulley,  which  sits  in  the  loop  formed  by  the 
passing  of  the  string  from  one  side  of  the 
cross-bar  down  and  up  the  tube  to  the 
other  side. 

It  follows  from  this  arrangement  that  a 
downward  motion  of  the  solenoid’s  core 
produces  an  upward  one  of  both  carbon 
points ; also  that  the  positive  carbon  moves 
twice  as  fast  as  the  negative  one,  and  that 
the  arc  is  maintained  always  at  the  same 
height.  From  the  different  rates  of  motion 
of  the  two  carbons  it  follows  that  a drawing 
down  of  the  core  separates  them,  and  its 
rising  allows  them  to  approach.  Projecting 
from  below  the  frame  carrying  the  wheel- 
work  is  seen  the  brake- wheel,  beneath  and 
close  to  which  is  the  brake,  a thin  strip  of 
brass,  which,  when  the  frame  has  fallen  a 
certain  space,  is  caught  by  a projection  on 
the  side  rod  and  prevented  from  moving 
down  further  with  the  frame,  thus  pressing 
on  the  wheel. 

The  action  of  the  lamp  is  as  follows  : — 
On  closing  the  circuit  the  carbon  points 
which  were  before  in  contact  are  separated 
by  the  drawing  down  of  the  core  consequent  upon 
the  action  of  the  current  in  the  main  coils.  This  also 
allows  the  brake  to  press  on  the  brake-wheel,  and  prevents 
the  carbons  from  running  together.  The  differential  action 
of  the  two  currents  circulating  in  the  coils  of  the  solenoid, 
i.e.  the  main  one  through  the  thick  wire,  and  the  shunt 
current  through  the  fine  wire,  causes  the  arc  to  adjust  itself 
to  the  proper  length.  When  this  from  any  cause  is  ex- 


Fig.  48. 


Plate  XII.  — crompton’s  arc  ramp— new  pattern1. 


The  New  Crompton  Lamp . 91 

ceeded,  its  increased  resistance  throws  more  current  through 
the  shunt  coils,  the  core  is  raised,  the  brake- wheel  liberated, 
and  the  carbons  approach.  If  the  arc  should  be  too  short, 
then,  the  shunt  current  being  diminished,  this  core  is  pulled 
down  and  the  arc  lengthened. 

The  D.  D.  Lamp.  (Plate  XII.  and  fig.  49.) 

The  latest  type  of  lamp  made  by  Messrs.  Crompton  and 
Co.  is  known  as  the  D.  D.  (double  differential)  lamp.  It 
also  is  the  joint  invention  of  Messrs.  Crompton  and  Crabb, 
and  shows  a marked  improvement  on  the  older  forms  in 
point  of  simplicity  of  construction  and  also  in  regulation. 
This  latter  is  effected  by  a brake-wheel  driven  by  the  rack 
rod  attached  to  the  upper  carbons,  as  in  the  1881  pattern, 
over  which,  however,  the  D.  D.,  in  common  with  the  K.  lamp, 
possesses  the  great  advantage  of  being  able  either  to 
increase  or  decrease  the  length  of  arc  as  may  be  necessary, 
whereas  the  older  lamp  could  only  decrease  it. 

In  the  D.D.  lamp  also  the  intermediate  gearing  between 
the  brake- wheel  ,and  the  pinion  driven  by  the  rack  rod  is 
dispensed  with.  Referring  to  Plate  XII.,  B and  B£  are 
the  rack  rods  carrying  the  positive  carbons.  Sliding  on 
each  of  these  is  a light  gun-metal  sleeve,  S SIa  carrying 
spindles,  to  which  are  attached  the  two  large  brake-wheels 
E EI?  and  between  them  the  pinion  which  gears  into  the 
racks.  To  each  side  rod  is  pivotted  a broad  lever,  L L,  at 
the  other  end  of  which  a chain  is  fastened,  connecting  it  to 
the  hollow  core  of  the  solenoid  vertically  above.  This 
solenoid  is  differential,  as  in  the  K lamp,  G being  the  shunt 
and  M the  main  coil,  and  has  its  core  partially  supported 
by  a spring  whose  tension  can  be  regulated  by  means  of 
the  screw  T.  Projecting  vertically  downwards  from  each 
sleeve  to  a distance  from  the  centre  of  the  spindle  about 
equal  to  the  radius  of  the  brake-wheels,  is  a stout  pin  or 
finger,  F Fx,  the  use  of  which  we  will  try  to  make  clear. 
Suppose  the  rack  rod  to  be  drawn  up ; then,  if  the  lever 
be  pulled  by  the  solenoid  above  the  horizontal  position,  the 
whole  weight  of  the  rod  and  carbon  is  supported  on  the 
edges  of  the  two  brake-wheels,  and  the  friction  of  them  on 
the  surface  of  the  levers  is  sufficient  to  prevent  their  revolu- 


92 


Electric  Lighting. 

tion  ; hence  this  rack  rod  cannot  run  down : but  if  the 
levers  be  below  the  horizontal,  then  the  weight  is  carried 
by  the  finger  projecting  from  the  sleeve,  as  shown  at  F, 
the  wheels  are  free  to  turn,  the  rack  runs  down,  and  con- 
tinues to  do  so  until  the  positive  and  negative  carbon  points 
come  in  contact.  Now  let  the  current  be  switched  on  : by 
its  passage  through  the  main  wire  of  the  solenoid,  the 
levers  are  raised,  striking  the  arc,  and  at  the  same  time 
applying  the  brake  to  the  wheels.  The  shunt  current  then 
flows,  and  the  arc  takes  its  proper  length.  If  this  becomes 
too  great,  the  increased  current  through  the  shunt  draws 
down  the  core  and  levers ; the  brake-wheels  are  left  free  to 
revolve  if  the  arc  shortens. 

On  the  other  hand,  if  the  carbon  points  be  too  close,  the 
levers  are  raised,  bringing  with  them  the  rack  rods  and  upper 
carbons.  Making  the  finger  projecting  from  one  sleeve 
longer  than  that  from  the  other,  determines  which  pair  of 
carbons  shall  begin  to  burn  first,  because,  on  switching  on, 
that  pair  which  has  the  longer  pin  will  be  the  last  to  break 
contact,  and  will  therefore  originate  an  arc  in  so  doing.  It 
will  be  easily  seen  from  Plate  XII.  that  on  the  core  being 
raised  the  lever  Lx  will  apply  the  brake  before  the  lever 
L does,  hence  it  may  be  said  that  the  rack  rod  B,  gets  a 
start  on  B ; its  carbon  points  are  separated  before  those  of 
B,  and  are  kept  a greater  distance  apart  until  the  latter  are 
consumed.  When  this  is  the  case,  the  rack  rod  Bx  is  pre- 
vented from  further  fall  by  a stop  and  can  no  longer  feed, 
hence  the  arc  will  lengthen,  the  shunt  current  will  increase, 
and  the  other  rod  Bx,  which  can  still  feed,  will  be  allowed  to 
descend  until  its  carbons  touch,  starting  a fresh  arc.  The 
core  is  raised  again,  the  fresh  arc  burning  instead  of  the 
old  one,  and  everything  goes  on  as  before. 

When  the  second  pair  of  carbons  has  burned  low,  the 
same  action  takes  place,  viz.  further  descent  of  the  rod  is 
prevented,  the  arc  lengthens,  the  shunt  current  increases, 
and  the  core  is  drawn  down,  but  lower  than  when  the  first 
pair  of  carbons  were  burned  out,  until  a copper  stud,  C, 
attached  to  it  makes  contact  with  another,  H,  connected  to 
the  negative  pole,  cutting  the  lamps  out  of  the  circuit  and 
introducing  an  equivalent  resistance. 


93 


The  New  Crompton  Lamp. 

The  connections  of  the  lamp  and  its  equivalent  resistance 
coil  are  shown  in  fig.  49.  The  current  entering  in  the 
direction  of  the  arrow  finds  two  paths  open,  the  one  through 
the  resistance  coil  R R and  insulated  contact  piece  C,  which 
for  the  time  being  is  resting  upon  H,  and  thus  on  to  the  next 
lamp ; and  the  other  through  the  switch  S.  which  we  suppose 
to  be  closed,  the  main  solenoid,  coils  M,  the  framework  of 
the  lamp,  the  positive  carbon,  the  negative  carbon,  and 


ultimately  out  by  H,  and  on  to  the  next  lamp.  The  latter 
portion  of  the  current  in  passing  round  the  core  mag- 
netizes it  and  draws  it  up,  thereby  breaking  contact  between 
C and  H. 

In  this  moment  the  current  has  only  one  path  open,  viz. 
that  through  the  switch  S,  main  solenoid,  and  carbons;  and 
since  the  whole  of  it  must  pass  through  the  main  solenoid, 
the  core  of  the  latter  is  definitely  drawn  up,  lifting  the  two 


94 


Electric  Lighting . 

rack  rods,  and  establishing  the  arc  between  one  pair  of 
carbons  as  explained  above. 

If,  through  the  falling  out  or  breaking  of  a carbon  or 
hanging  up  of  the  rack  rods,  the  current  should  be  in- 
terrupted, then  the  core  of  the  solenoid  will  instantly  fall, 
and,  by  bringing  C and  H again  into  contact,  open  to 
the  current  the  former  path  through  the  resistance  coil. 
In  this  manner  an  accident  to  one  lamp  does  not  affect 
the  other  lamps  burning  in  series  with  it  on  the  same 
circuit. 

The  same  result  will  follow  if  the  current  be  interrupted 
through  the  opening  of  switch  S,  and  the  lamp  thereby 
be  switched  out  of  circuit. 

When  fitted  with  the  full  length  of  19  § inches  of  carbon 
13  m.m.  in  diameter,  the  lamps  will  burn  from  12  to  16  hours, 
according  to  the  current  passing,  which  may  vary  from 
6 to  28  amperes,  the  light  varying  from  850  to  6500 
candles.* 

The  electro-motive  force  required  is  50  to  60  volts. 
Adaptation  op  the  Ceompton  D.  D.  Lamp  to  Alteenating 

CUEEENTS. 

On  trying  the  above  lampwith  alternating  currents,  I found 
that  at  first  it  did  not  burn  satisfactorily.  An  examination 
of  the  currents  circulating  in  the  different  parts  showed  the 
curious  fact  that  the  current  in  the  shunt  coil,  instead  of 
varying  inversely  as  the  current  in  the  main  wire,  varied 
directly  with  it,  and  of  course  the  lamp  did  not  regulate. 

The  reason  of  this  was  that  the  coils  of  thick  and  thin 
wire,  having  a long  iron  core  common  to  both,  acted  like  an 
induction  coil,  and  the  alternating  current  in  the  thick  wire 
induced  currents  in  the  fine  wire,  whose  strengths  were  of 
course  proportional  to  its  own  strength,  and  which  quite 
overpowered  the  shunt  current. 

To  get  over  the  difficulty,  I cut  the  core  into  two  halves, 
and  connected  them  by  a brass  rod  equal  in  length  to  one 
of  the  halves.  I then  turned  the  coils  over  so  that  the  thick 

* The  above  candle-power  being  measured  at  an  angle  of  30°  below 
the  horizontal  line  cutting  the  arc  itself. 


The  Brush  Lamp. 


95 


wire  coil  was  at  the 
bottom  and  the  thin 
at  the  top  (fig.  50). 

A comparison  of  figs. 

49  and  50  shows  the 
nature  of  the  alteration. 

On  again  trying  the 
lamp,  I found  that  the 
induction  coil  effect  had 
ceased,  and  that  the 
lamp  regulated  per- 
fectly. 

Mounting. 

Fig.  51  shows  a Crompton  lamp  mounted 
in  an  ornamental  case. 


Fig.  51. 


Fig.  52. 


The  Brush  Lamp.  (Plate  XIII.) 


In  the  Brush  lamp,  shown  in  Plate  XIII.  and  iu  fig.  52, 


96 


Electric  Lighting . 

there  are  also  two  pairs  of  carbons,  and  the  mechanism  is  so 
arranged  that  when  the  current  is  started  the  arc  is  formed 
between  one  pair,  which  continue  to  burn  till  they  are  con- 
sumed, when  the  current  is  instantly  transferred  to  the 
second  pair.  This  lamp  therefore  can  also  be  made  to  burn 
all  night  without  haying  very  long  carbons. 

The  small  magnet  seen  in  Plato  XIII.  works  a “ cutout 33 
similar  in  principle  to  that  already  described  in  the  Crompton 
lamp.  The  large  magnet  both  makes  the  arc  and  regulates 
the  descent  of  the  carbon. 

The  magnet  is  wound  with  two  wires,  a thick  and  a fine 
one.  The  thick  wire  is  in  the  main  circuit,  and  the  fine 
wire  forms  a shunt  to  the  arc.  The  wires  are  so  connected 
that  the  currents  in  them  circulate  round  the  magnet  in 
opposite  directions,  so  that  the  strength  of  the  magnet 
depends  only  on  the  difference  between  the  main  current 
and  the  shunt  current.  The  action  of  the  main  current  being 
made  the  more  powerful  of  the  two,  the  main  current  tends  to 
increase  the  magnetism,  and  the  shunt  current  to  decrease  it. 

The  cores  of  the  magnet  are  tubular,  and  two  iron  bars  are 
attached  to  the  armature,  which  are  sucked  into  the  magnet 
as  the  current  gets  stronger.  This  arrangement  enables  a, 
play  of  some  two  inches  to  be  given  to  the  armature. 

When  the  armature  is  raised  it  lifts  the  upper  carbons  by 
means  of  the  clutch  shown  in  detail  at  the  bottom  of  Plate  XIII. 
Each  of  the  sliding  rods  carrying  the  carbon  passes  through 
a loose  washer,  which  does  not  grip  it  as  long  as  the  washer 
is  level.  One  side  of  the  washer  enters  a notch  in  a strip 
of  brass  attached  to  the  armature.  When  the  armature  is 
raised,  one  side  of  the  washer  is  lifted  by  it,  and  the  washer, 
being  tilted,  grips  the  rod,  and  lifts  the  carbon  with  it. 

In  order  to  avoid  sudden  jerks,  a cylinder  full  of  glycerine 
is  attached  to  the  armature,  and  a piston  working  in  it  is 
fixed  to  the  upper  part  of  the  lamp,  as  shown  in  Plate  XIII. 
The  armature  is  thus  compelled  to  move  slowly  and  smoothly. 
A similar  arrangement  is  applied  to  the  carbons.  The  rods 
carrying  them  are  made  tubular  and  filled  with  glycerine, 
and  pistons  are  fixed  to  stout  wires  at  the  top  of  the  tubes 
seen  in  Plate  XIII. 


Plate  XIII. — brush’s  arc  lamp. 


• ■ 1 


■y 


The  Brush  Lamp. 


97 


Before  the  current  passes,  both  carbons  sink  till  they  are 
in  contact  with  the  lower  carbons.  When  the  current  com- 
mences it  at  first  passes  partly  by  the  “ cut-out/'  and  partly 
through  the  magnets  and  carbons.  The  “ cut-out  " magnet 
breaks  the  short-circuit,  and  the  whole  current  passes  by  the 
magnets  and  carbons.  The  main  current  being  much  stronger 
than  the  shunt,  the  armature  is  lifted,  raising  both  carbons. 

A reference  to  Plate  XIII.  shows  us  that  the  lower  edge  of 
the  left-hand  slit  is  at  a higher  level  than  that  of  the  right- 
hand  one.  The  right-hand  carbon  is  therefore  less  raised 
than  the  left-hand  one,  and  the  arc  forms  itself  between  the 
right-hand  carbons. 

As  the  resistance  of  the  arc  increases,  the  main  current, 
which  tends  to  magnetize  the  magnet,  diminishes,  and  the 
shunt  current,  which  tends  to  demagnetize  it,  increases, 
and  so  the  magnetism  diminishes,  and  the  armature  sinks. 
As  soon  as  it  has  sunk  a little,  the  right-hand  washer  becomes 
level,  and  allows  the  carbon- holder  to  slip  through  it.  The 
arc  becoming  shorter,  the  power  of  the  magnet  increases,  and 
the  armature  is  raised,  and  the  carbon  again  locked. 

During  this  adjustment  the  armature  never  sinks  far 
enough  to  release  the  left-hand  washer.  The  adjustment  is 
repeated  at  short  intervals  until  the  right  hand  carbon  is 
consumed,  say  in  about  eight  hours  from  the  time  when  the 
lamp  was  lighted. 

When  the  carbon  is  consumed  the  holder  is  stopped  by  a 
collar,  and  the  current  through  the  carbons  ceases  for  an 
instant,  and  the  cut-out  armature  falling,  the  lamp  is  short- 
circuited. 

The  left-hand  carbon  then  instantly  falls,  and  comes  into 
contact  with  its  lower  carbon.  The  circuit  through  the 
lamp  being  re-established,  the  cut-out  armature  is  again 
lifted,  the  carbon  is  raised  so  as  to  form  a new  arc,  and  the 
regulation  goes  on  in  the  same  manner  as  before  until  the 
second  carbon  is  entirely  consumed.  At  the  instant  of  change 
of  the  current  from  one  pair  of  carbons  to  the  other  the  lamp 
is  extinguished  for  something  less  than  a second. 

The  ordinary  lamps  used  for  street  lighting  burn  carbons 
of  ° millims.  diameter,  and  are  said  to  use  10  amperes  of 

n 


98 


Electric  Lighting . 

current,  and  to  require  an  E.M.F.  of  about  50  volts. 
H.P.  consumed  in  them  would  be,  by  equation  (7), — 


H.P. 


50  x 10 
746 


•67. 


These  lights  are  probably  of  about  800-candle  power, 


The 


Fig.  53. 


The  Brush  Company  have  been  very  successful  in  work- 
ing a large  number  of  their  lamps  in  series  on  one  circuit. 
Forty  on  one  circuit  are  in  regular  use  in  Cheapside. 


The  J ablochkoff  Candle. 

Fig.  53  represents  a Brush  lamp  mounted  on  e 
lamp-post. 

Brush  lamps  are  also  made  with  larger  carbons, 
taking  more  power.  One  has  been  constructed 
with  carbons  3^  inches  diameter,  requiring  40  H.P., 
and  giving  a light  said  by  the  company  to  be  equal 
to  150,000  candles.  This  is  by  far  the  largest  electric 
lamp  of  any  kind  which  has  yet  been  made. 

The  Jablochkoff  Candle. 

In  the  Jablochkoff  candle  (fig.  54)  the  two  carbon 
rods  are  placed  side  by  side,  and  are  separated  by 
a strip  of  plaster  composed  of  kaolin  and  other 
ingredients. 

The  current  passes  up  one  carbon  and  down  the 
other,  and  the  arc  is  formed  at  the  top.  The  whole 
burns  downwards  like  a candle.  The  candles  are 
tipped  with  a paste  made  of  powdered  carbon  and 
gum.  When  the  current  is  first  started  it  passes 
through  this  paste,  which,  becoming  incandescent, 
rapidly  burns  away,  and  leaves  an  arc  formed  be- 
tween the  poles.  In  case  of  the  current  being  acci- 
dentally interrupted  for  an  instant,  the  candles  go 
out,  and  do  not  re-ignite  themselves. 

In  the  Jablochkoff  candle  the  length  of  the  arc 
remains  constant,  but  its  resistance  is  constantly 
varying  with  every  impurity  or  change  of  density  in 
the  plaster.  The  light  is  therefore  extremely  un- 
steady. The  extreme  simplicity  of  the  Jablochkoff 
candle,  and  the  absence  of  machinery,  are  however 
a certain  recommendation  to  it.  It  is  of  consider- 
able historical  interest,  as  the  lighting  of  the  Avenue 
de  TOpera  in  Paris  in  1878  by  Jablochkoff  candles 
first  demonstrated  the  possibility  of  street  lighting 
by  electricity. 

The  Jablochkoff  candle  can  only  be  used  with 
alternating  currents,  as  it  is  necessary  that  the 
carbons  should  consume  equally. 

h 2 


99 

street 


Fig.  45. 


IOO 


Electric  Lighting. 


CHAPTER  VIII* 

CARBONS  FOR  ARC  LAMPS. 

Davy's  first  experiment  with  the  electric  arc  was  carried  out 
with  pieces  of  wood  charcoal  as  electrodes,  but  it  was  at 
once  seen  that  electrodes  formed  of  such  a soft  material 
could  not  be  of  much  practical  use,  as  they  burned  away  too 
rapidly,  and  gave  off  coruscations  of  dangerous  sparks.  It 
may  be  here  mentioned  that,  seventy  years  later,  Gaudoin,  at 
Paris,  again  reverted  to  the  use  of  rods  of  wood  charcoal,  the 
density  of  which  he  increased  to  any  required  extent  by 
filling  up  the  pores  with  various  hydro-carbons,  alternately 
soaking  the  rods  in  liquid  hydro-carbon,  and  firing  them 
until  they  gave  a metallic  ring.  But  for  many  years  the 
carbon  electrodes  used  for  all  experiments  with  the  electric 
light  were  strips  sawn  from  the  graphitic  deposits  found 
in  gas  retorts.  Foucault  has  the  credit  of  introducing  these. 

The  carbon  electrodes  as  now  used  are  the  result  of  the 
experimental  researches  of  Staite,  Laccasagne,  and  Thiers, 
Archereau,  Carre,  Gaudoin,  and  others.  Most  of  the  makers 
observe  a certain  amount  of  secrecy  in  regard  to  their  pro- 
cesses of  manufacture,  but  the  general  features  of  it  are 
well  known  to  be  as  follows  : — 

Coke  or  graphite  is  finely  powdered  and  washed  in 
alkaline  cells  to  get  rid  of  the  silica  and  earthy  impurities, 
after  which  it  is  ground  in  a pug-mill,  with  sufficient 
syrupy  or  tarry  hydro -carbon  to  agglutinate  it  into  a stiff 
paste.  This  paste  is  then  pressed  into  rods  of  the  required 
form  by  being  forced  through  moulds  or  dies. 


* I have  to  thank  Mr.  Crompton  for  this  chapter. 


IOI 


Carbons  for  Arc  Lamps . 

Sometimes  tlie  pressure  is  applied  endways,  the  paste 
being  forced  out  in  a continuous  rod  of  the  proper  thickness, 
and  cut  off  into  lengths  as  required.  Another  plan  is  to 
apply  the  pressure  sideways,  the  dies  being  divided  longitu- 
dinally into  two  parts.  In  the  latter  case,  several  rods  are 
usually  pressed  at  one  time.  The  rods  thus  formed  are  care- 
fully dried,  and  afterwards  fired  in  kilns,  having  been 
previously  packed  in  air-tight  boxes,  and  embedded  in  coke 
dust.  After  one  firing  they  are  generally  found  to  be  porous, 
and  require  soaking  in  syrup  and  a second  time  firing. 
Some  of  the  makers  repeat  this  process  more  than  once. 

The  manufacturers  who  during  the  last  few  years  have 
done  most  to  perfect  carbon  electrodes  have  been — Carre, 
Sautter-Lemmonier,  and  Mignon-Rouart,  in  France;  Siemens 
in  Germany;  Gray,  Hedges,  and  Johnson  and  Phillips  in 
England. 

The  following  points  are  aimed  at  in  the  production  of  a 
perfect  carbon  for  arc  lighting : — 

1.  Freedom  from  all  matter  other  than  carbon. 

2.  Regularity  of  density. 

3.  Mechanical  perfection  of  form. 

4.  Low  electrical  resistance. 

Purity. 

It  is  not  sufficient  that  the  coke  or  other  powdered  carbon 
from  which  the  rods  are  made  should  be  in  the  first  instance 
free  from  earthy  or  metallic  impurities,  but  a great  deal 
depends  on  the  care  taken  in  subsequent  processes  of  manu- 
facture to  insure  that  the  volatile  gases,  most  of  them  hydro- 
carbons, are  thoroughly  expelled  during  the  process  of 
firing.  The  reasons  for  this  are  as  follows  : — 

Carbon  being  the  most  refractory  of  any  known  substance, 
disintegrates  to  pass  across  the  arc  at  the  highest  possible 
temperature,  and  as  the  whiteness  of  the  colour,  as  well  as 
the  amount  of  the  light  given  by  the  electric  arc,  depends 
entirely  upon  the  temperature  of  the  arc,  it  is  evident  that 
any  admixture  of  substance  other  than  carbon  will,  by  lower- 
ing the  mean  temperature  at  which  the  electrode  is  dis- 
integrated, tend  to  diminish  the  amount  of  light  given. 


102 


Electric  Lighting. 

Observations  taken  with  the  spectroscope  show  that  at 
the  times  when  the  arc  distils  itself  free  from  all  foreign 
salts,  so  that  we  have  in  the  spectrum  only  the  characteristic 
carbon  lines,  the  colour  of  the  light  is  most  intensely  white, 
and  the  amount  of  light  is  at  a maximum  ; and  it  is  almost 
certain  the  temperature  is  also  at  a maximum.  At  the 
same  time  it  is  observable  that  the  conductivity  of  the 
stream  of  matter  passing  across  from  one  electrode  to  another 
is  at  a minimum,  hence  if  the  difference  of  potential  at 
the  two  sides  of  the  arc  is  maintained  constant,  the  arc 
will  be  shortest  at  the  times  that  the  carbon  is  purest,  and 
the  action  being  then  confined  to  the  smallest  possible  space, 
will  be  intensified  in  proportion,  resulting  in  greatly  increased 
light  and  economy.  The  addition  of  the  smallest  portion  of 
any  material  which  volatilizes  at  a lower  temperature  than 
carbon  itself,  increases  the  length  of  the  arc  with  the  results 
the  reverse  of  those  above  mentioned. 

Gaseous  impurities,  the  chief  offenders  being  hydro- 
carbons, are  peculiarly  annoying  in  this  respect.  "When  they 
are  present  to  any  extent,  they  always  break  out  at  irregular 
intervals  as  blowers  of  gas,  which  are  comparatively  of  high 
conductivity.  These  jets  play  around  the  crater  proper  in 
a most  irregular  manner,  and  are  the  chief  cause  of  the 
flickering  so  often  complained  of  in  arc  lighting.  It  is  quite 
common  to  see  the  arc  start  from  a point  very  high  up  on 
the  cone  outside  the  crater,  and  forming  a curved  and  rapidly- 
moving  jet  towards  a point  on  the  cone  of  the  negative 
carbon.  At  such  moments  the  light  is  found  diminished  to 
one-third  or  one- quarter  of  its  normal  brilliancy,  and  as  the 
lamp  adjusts  itself  to  the  new  conditions  by  automatically 
lengthening  the  arc,  a vibratory  or  reciprocating  action  is 
set  up  in  the  lamp,  which  greatly  intensifies  the  mischief. 

Silica  and  other  earthy  matter  when  present  in  carbon,  in 
addition  to  lowering  the  arc  temperature,  form  a more  or 
less  bulky  ash,  which  falls  down  into  the  surrounding 
globes,  and  is  extremely  unsightly.  When  continuous  cur- 
rents are  used,  this  ash  accumulates  on  the  negative  elec- 
trodes, and  thus  produces  ugly  and  otherwise  objectionable 
shadows. 


Carbons  for  Arc  Lamps. 


103 


Regular  Density. 

This  has  been  attained  much  more  perfectly  of  late 
years  since  the  introduction  of  dies  split  longitudinally, 
so  that  a side  pressure  equal  in  extent  throughout 
the  full  length  of  the  rod  can  be  applied.  Rods  made 
by  end  pressure  very  frequently  have  the  ends  more  or 
less  dense  than  the  middle  portion  ; consequently,  when  such 
rods  are  used,  the  lamps  burn  quite  differently  when  first 
started,  and  when  the  carbons  are  nearly  burned  out. 

A defect,  which  shows  itself  more  particularly  with  carbons 
used  for  continuous  currents,  and  which  probably  is  due 
to  irregularity  in  density,  is  that  the  point  of  the  positive 
carbon,  instead  of  forming  itself  into  a crater  of  regular  con- 
cavity, forms  a number  of  small  craters  or  facits  inside  one 
general  crater.  This  irregularity  is  always  accompanied 
by  noise  and  unsteadiness  when  burning. 

It  seems  as  if,  although  the  density  of  the  rod  may  be  fairly 
regular  throughout,  yet  that  the  whole  is  made  up  of  a 
honeycomb  of  dense  carbon  filled  in  with  softer  carbon,  the 
action  of  the  current  being,  as  it  were,  to  excavate  the 
soft  carbon,  leaving  the  hard  carbon  ridges  prominent. 

Carre,  Siemens,  and  others  have  attempted  to  get  over 
this  difficulty  by  purposely  varying  the  density  of  the  rod. 
That  is  to  say,  they  have  put  a soft  core  into  a hard  carbon, 
so  that  the  current  itself  would  always  excavate  the  soft 
core  slightly  in  advance  of  the  hard  carbon  exterior.  This 
insures  that  the  crater  be  kept  truly  concentric  with  the 
rod,  and  certainly  it  has  given  the  desired  effect.  Otherwise 
this  defect  of  irregular  local  density  could  be  got  over  by 
increased  care  in  the  grinding  and  pugging  of  the  carbon 
paste  prior  to  the  pressing. 

Mechanical  Perfection  of  Form. 

Little  need  be  said  on  this  head.  It  is  evident  that 
modern  requirements,  which  insist  on  arc  lamps  burning  for 
many  hours  at  a stretch  without  renewal  of  the  electrodes, 
necessitate  long  rods,  and  these  must  be  perfectly  cylin- 
drical, of  equal  diameter  throughout,  and  perfectly  straight. 


104 


Electric  Lighting. 

The  longer  the  rods  are  made,  the  more  difficult  it  is  to 
preserve  their  straightness  through  the  many  processes  of 
drying,  firing,  soaking,  &c. 

Some  of  the  densest,  purest,  and  most  regular  carbons  in 
the  market  are  great  offenders  in  this  respect,  and  conse- 
quently are  quite  useless  for  long  burning  lamps.  If  the 
rods  are  not  perfectly  cylindrical  and  of  equal  diameter 
throughout,  it  is  very  difficult  to  make  good  contact  with  the 
carbon-holders,  and  as  a result  the  carbons  heat  at  the  point 
where  they  are  nipped  by  the  holders,  even  to  such  an 
extent  as  to  char  away,  and  thus  become  loose  and  fall  out. 
Any  irregularity  at  this  point  makes  it  next  to  impossible 
for  the  attendant  changing  the  carbons  to  get  the  two  rods 
into  line  with  each  other. 

* Electrical  Resistance  of  Carbon  Rods. 

This  is  a matter  of  greater  importance  than  might  be  at 
first  imagined.  When  many  arc  lamps  are  burned  in  series, 
if  the  electrical  resistance  of  the  carbons  themselves  is  high, 
it  becomes  a serious  matter  to  deal  with.  For  instance, 
with  forty  arc  lamps  in  series,  the  resistance  of  the  carbon 
rods  only,  when  the  whole  of  the  lamps  are  newly  replenished, 
will  be,  under  the  most  favourable  conditions,  twenty  ohms, 
and  this  becomes  reduced  to  two  or  three  ohms  when  the 
rods  are  burned  down  short. 

The  purest  and  densest  carbons  are  also  found  to  have  the 
lowest  resistance,  but  in  such  a case  as  that  above  men- 
tioned, or  in  fact  wherever  more  than  sixteen  lamps  are  used 
in  series,  it  is  found  necessary  to  reduce  the  resistance  by 
electro-plating  the  carbons  with  copper  or  nickel.  This 
copper-plating  presents  serious  disadvantages ; one  of  them 
being  that,  when  direct  currents  are  used,  the  copper  on 
the  negative  rods  is  not  burned  away  under  the  action  of 
the  arc  as  fast  as  the  carbon  itself.  It  therefore  stands  up 
in  the  form  of  a ragged  fringe  round  the  negative  rod, 
which  throws  a large  star-shaped  shadow  on  the  floor  beneath. 
It  is  also  believed  that  the  volatilized  copper  which  is  always 
present  in  the  atmosphere  when  copper-coated  carbons  are 
used,  although  harmless  in  the  open  air,  would  be  dangerous 
to  health  in  confined  workshops.  Hedges  succeeded  in 


Carbons  for  Arc  Lamps . 105 

plating  his  carbons  with  iron,  which  in  one  way  got  over 
this  difficulty , but  he  met  with  a fresh  one,  for  it  was  im- 
possible to  keep  the  covering  from  rusting  unless  it  was 
protected  with  some  varnish,  which  had  to  be  scraped  off  at 
the  point  where  the  carbon  entered  the  holder.  De  Hamel 
patented  a process  of  inserting  an  iron  wire  up  the  centre  of 
carbon  rods,  the  idea  being  to  diminish  the  resistance,  it 
being  also  found  that  the  volatilizing  of  the  iron  did  not  affect 
the  colour  of  the  light ; but  there  is  no  doubt  that,  for  the 
reasons  above  given,  it  lengthened  the  arc,  and  diminished 
its  economical  efficiency. 

At  any  rate,  this  process  has  so  far  not  proved  an  eco- 
nomical success. 


The  Size  of  Carbon  Electrodes. 

The  economical  efficiency  on  the  one  hand,  and  the  steadi- 
ness of  the  light  on  the  other,  depend  very  greatly  on  the 
diameter  of  the  carbon  electrode  employed  with  a given 
current.  Within  certain  limits  the  one  is  in  inverse  ratio  to 
the  other.  That  is  to  say,  as  we  decrease  the  diameter  of 
the  carbon,  we  increase  the  amount  of  light  from  a given 
current,  and  decrease  its  steadiness.  The  diameters  most 
commonly  used  have  been  as  follows  : — 


For  currents  from  7 — 12  amperes 


33 

J) 

39 


12—18 

18—25 

25—40 


r> 


40  upwards 


9 m.in.#  to  11  m.m.  diam. 
11  m.m.  ,,  13  m.m.  „ 

13  m.m.  „ 15  m.m.  „ 

15  m.m.  „ 18  m.m,  „ 

18  m.m.  „ 20  m.m.  „ 


It  is  extremely  difficult  to  obtain  carbons  above  20  m.m.  of 
sufficient  homogeneous  texture  to  give  a steady  light  with 
lamps  having  automatic  regulation.  All  carbons  above  this 
diameter  are  used  with  the  large  arcs  as  search  lights  for 
military  and  naval  purposes,  and  most  commonly  worked  by 
non-automatic  hand  regulators,  and  as  all  the  best  modern 
projectors  are  fitted  with  an  arrangement  for  viewing  the 
arc  from  the  side,  the  attendant  in  charge  can  quickly  set 
right  any  irregularity  caused  by  change  of  density. 

If  the  above  rules  as  to  diameter  proportionate  to  current 
are  departed  from,  the  following  results  take  place  : — 

* One  m.m.  (millimetre)  is  practically  equal  to  5L  inch. 


io6 


Electric  Lighting . 

If  a carbon  of  too  small  diameter  is  used,  the  positive 
carbon  cores  away  so  rapidly  as  to  cut  down  the  sides  of  the 
crater,  and  the  intensely-heated  portion  of  the  carbon 
extends  outside  the  walls  of  the  crater  proper ; in  this  way 
a great  amount  of  the  light  which  ordinarily  would  be  thrown 
downwards  on  to  the  surface  required  to  be  illuminated, 
will  be  wasted  in  the  upper  part  of  the  spherical  angle.  In 
addition  to  this  loss,  there  is  great  danger  of  the  carbon 
becoming  red-hot  from  end  to  end,  and  thus  wasting  away 
like  a candle  in  a hot  oven.  Again,  the  light  also  becomes 
very  unsteady ; the  arc  does  not  pass  steadily  between  the 
points  of  the  cone  and  the  crater  proper,  and  the  lamp  burns 
for  a shorter  time  than  it  ought  to  do. 

On  the  other  hand,  if  carbons  of  too  great  diameter  are 
used,  they  do  not  point  properly,  and  consequently,  the  angle 
of  the  lower  carbon  being  very  obtuse,  throws  down  a 
large  shadow.  A great  part  of  the  electric  energy,  instead 
of  being  utilized  in  producing  an  intense  temperature  at 
the  crater,  is  wasted  in  heating  the  external  portion  of 
the  massive  carbon  to  red  heat. 

Nevertheless  there  is  no  doubt  that  the  use  of  large 
carbons  has  greatly  extended  during  the  last  few  years. 
Although  energy  is  wasted,  and  consequently  less  light  is 
afforded  by  a given  current,  there  is  a great  increase  in 
steadiness,  and  the  lamps  burning  for  longer  hours  do  not 
require  so  much  attention. 

The  following  table  gives  the  result  of  experiments  on 
various  carbons  : — 


Illuminating  Power  per  Electrical  H.P.  of  13  m.m.  Carbons  of 
Different  Makers. 

Currents  from  15  to  20  Amperes. 


Name  of  Maker. 

Candle-Power 
per  H.P. 

Difference  of 
Potential 
either  side  of  Arc. 

Remarks. 

Siemens  (cored)  pos.  7 
Carre  (cored)  neg.  ) 

4270 

47-4 

Mean  of  4 Readings. 

Siemens  (cored)  . 

3514 

46-9 

„ 12  „ 

Barnsley  Co. 

3500 

49-8 

„ 4 „ 

Johnson  & Phillips  . 

2986 

39*7 

3 >> 

Sautter  & Lemonnier . 

2920 

48*0 

„ 6 » 

Carre  (not  cored) 

2773 

45-1 

» 6 „ 

Silvertown  (Grays) 

2580 

46-1 

jj  2 „ 

Carre  (cored) 

1972 

42-8 

5?  5 >’ 

Carbons  for  Arc  Lamps.  107 


C urrents  from  5 to  15  Amperes. 


Name  of  Maker. 

Candle-Power 
per  H.P. 

Difference  of 
Potential 
either  side  of  Arc. 

Remarks. 

Siemens  (cored)  pos.  ) 
Carre  (cored)  neg.  $ 

3564 

54.0 

Mean  of  4 Readings. 

Barnsley  Co. 

3010 

54-2 

„ 4 

Silvertown  (Grays) 

2715 

46-9 

„ 2 „ 

Carre  (not  cored) 

2650 

40-75 

»>  2 ,, 

Sautter  & Lemonnier 

2442 

48-0 

4 

55  ^ J5 

Johnson  & Phillips  . 

1820 

39-75 

» 2 „ 

Carre  (cored) 

1667 

46-2 

„ 4 „ 

The  current  was  measured  by  a Sir  William  Thomson  Current 
Galvanometer. 

The  volts  were  measured  by  a Crompton-Kapp  Potential  Indicator. 

The  candle-power  was  measured  by  a Sugg’s  Patent  Bunsen  Photo- 
meter with  a 100-inch  bar  and  a 16-candle  standard  Argand  Gas 
Burner. 

The  photometric  measurements  were  taken  direct  from  the  arc,  the 
lamps  being  inclined  30°  from  the  vertical  position,  so  that  the  measure- 
ments are  equivalent  to  those  taken  30°  below  the  horizontal  line 


Illuminating  Power  per  Electrical  H.P.  of  Siemens’  Cored 
Carbons  of  Yarying  Diameters. 


Amperes. 

13  m.m. 

Candles.  Volts. 

13  m.m. 

Candles.  Volts. 

11  m.m. 

Candles.  Volts. 

5 to  14 

2660 

60-0 

2278 

48-0 

2549 

52-2 

14  „ 18 

4400 

39-0 

3514 

46-9 

— 

— 

18  „ 25 

5263 

42-75 

3637 

42*9 

— 

— 

io8 


Electric  Lighting. 


CHAPTER  IX. 

MAGNETS  AND  ELECTRO-MAGNETIC  INDUCTION. 


Magnets — Preliminary  Note. 

Magnets  are  of  two  kinds,  either  steel  “ permanent  mag- 
nets,” which  only  differ  in  size  and  strength  from  the  little 
magnets  of  the  toy  shops,  or  ce  electro-magnets,”  which  con- 
sist of  a core  of  soft  iron  wound  with  a quantity  of  insulated 
wire,  and  only  become  magnetic  when  a current  of  elec- 
tricity is  sent  through  the  wire,  the  polarity  and  strength  of 
the  magnets  depending  on  the  direction  and  strength  of  the 
currents. 

If  the  iron  core  of  an  electro-magnet  is  removed,  the 
helix  of  wire  when  traversed  by  a current  still  exhibits 
magnetic  properties,  much  feebler  in  degree,  but  the  same 
in  kind  and  direction,  as  those  of  the  electro-magnet. 

A coil  or  ring  of  wire  carrying  a current  may  therefore  be 
regarded  as  an  electro-magnet,  whose  polarity  is  the  same 
as  it  would  have  been  if  an  iron  core  had  been  inserted. 

Relation  between  Polarity  and  Direction  oe  Current. 

If  in  an  electro-magnet  we  imagine  a man  to  be  swimming 
in  the  current  down-stream  and  looking  at  the  core , the  north 
' pole  will  be  on  his  left  hand. 

By  the  north  pole  I mean  the  one  which  would  repel  the 
marked  end  of  a compass-needle,  or  which  would  point 
northwards  if  the  magnet  were  freely  suspended  in  a hori- 
zontal position. 


Electro- Magnetism . 


109 


Electro-magnetic  Attractions  and  Repulsions. 

Magnetic  poles  of  the  same  name  repel  each  other,  those 
of  opposite  names  attract.  Currents  in  the  same  direction 
attract  each  other,  those  in  opposite  directions  repel. 

If  the  force  between  the  poles  of  two  electro-magnets  is 
repulsive,  the  force  between  their  coils  when  the  cores  are 
removed  is  also  repulsive,  and  vice  versa. 

Let  figs.  55  and  56  each  represent  two  electro-magnets 
placed  side  by  side. 

In  fig.  55  the  poles  are 
dissimilar  and  will  attract* 
and  we  see  that  the  cur- 
rents in  neighbouring 
wires  are  in  the  same  di- 
rection, and  therefore  will 
attract  also. 

In  fig.  56  the  poles  are 
similar,  and  the  currents 
in  neighbouring  wires  are 
in  opposite  directions,  and 
therefore  both  poles  and 
wires  repel  each  other. 

If  in  each  figure  we  imagine  the  right-hand  magnet  to 
be  turned  over  so  as  to  face  the  left-hand  one,  we  see  that 
the  forces  will  be  of  the  same  signs  as  before. 


Fig.  56. 


Lines  op  Magnetic  Force. 

Every  magnet,  whether  it  is  a permanent  magnet  or  an 
electro-magnet,  or  the  imaginary  magnet  to  which  a coil 
of  wire  carrying  a current  is  equivalent,  may  be  considered 
as  having  lines  of  force  emanating  from  its  poles. 

These  lines  of  force  never  have  a free  end. 

If  a line  of  force  starts  from,  for  instance,  the  N.  pole  of 
a magnet,  its  other  end  must  be  in  a S.  pole  somewhere. 
This  S.  pole  may  either  be  the  S.  pole  of  the  same  magnet, 
or  a S.  pole  of  another  magnet,  or  a S.  pole  induced  by 
the  magnet  itself  in  a neighbouring  piece  of  iron.  Thus 
the  directions  and  curvatures  of  the  lines  of  force  of  a 


I IO 


Electric  Lighting . 

magnet  are  affected  by  any  pieces  of  iron,  or  by  any  magnets 
that  may  be  near  it. 

Magnetic  Field. 

The  space  round  a magnet  is  called  a magnetic  field. 
The  strength  of  a magnetic  field  may  be  conveniently  ex- 
pressed by  the  number  of  lines  of  force  which  pass  through 
it.  The  strength  of  the  field  at  any  point  may  be  expressed 
by  the  number  of  lines  of  force  passing  through  a unit 
of  area  placed  at  that  point.  Thus,  where  the  force  is 
strong  the  lines  of  force  may  be  said  to  be  crowded  to- 
gether, and  where  it  is  weak  to  be  thinly  distributed.  Just 
in  the  same  way  the  density  of  a crowd  at  any  point  might 
be  expressed  by  the  number  of  people  standing  on  a square 
yard  of  ground  at  that  point. 

We  must  bear  in  mind,  however,  that  the  lines  of  force 
have  no  breadth , and  that  consequently  there  is  no  limit  to  the 
number  that  can  jpass  through  a given  area.  This  fact  is  of 
great  importance  in  the  construction  of  dynamo  machines. 

Magnetic  Induction. 

A magnetic  pole  at  rest  near  a piece  of  soft  iron  induces 
an  opposite  pole  to  itself  at  the  end  of  the  iron  nearest  to 
it,  and  a pole  similar  to  itself  at  the  far  end. 

Experimental  tracing  of  the  Lines  of  Force  of  a 
Magnet.  (Plate  XIY.) 

The  direction  of  the  lines  of  force  of  a magnet  may  be 
accurately  determined,  and  the  relative  number  which  pass 
through  each  portion  of  the  field  may  be  approximately 
determined,  by  the  following  method  : — 

Let  a stout,  highly-glazed  card  be  laid  on  the  poles  of 
the  magnet,  and  let  a few  iron  filings  be  dusted  on  to  it 
from  a sieve,  the  card  being  gently  tapped  during  the  whole 
time.  The  filings  will  arrange  themselves  along  the  lines 
of  force,  and  will  be  thickest  where  the  field  is  strongest, 
i.e.  there  will  be  a greater  number  of  lines  of  filings  where 
there  are  a greater  number  of  lines  of  force. 

If  it  is  required  to  preserve  the  curves,  a sheet  of  paper, 


Plate  XIY. — lines  or  magnetic  fobce. 


Electro-Magnetic  Induction.  1 1 1 

with  its  under  surface  gummed,  may  be  laid  on  them  so  that 
the  filings  may  adhere  to  it. 

Plate  XIY.  represents  some  lines  of  force  determined  by 
Faraday. 

Another  method  of  tracing  out  lines  of  force  and  mea- 
suring the  intensity  of  magnetic  fields  depends  on  electro- 
magnetic induction,  and  will  be  described  later  on.  See 
page  117. 

Continuity  op  Physical  Change. 

All  physical  changes  of  whatever  hind  are  continuous , 
i.e.  if  a body  is  in  one  state  at  one  time  and  in  another  state 
at  another  time,  we  know  that  between  those  times  it  must 
have  passed  through  all  intermediate  states. 

For  instance,  if  a body  has  a temperature  of  10°  at  one 
time  and  of  20°  at  another  time,  then  we  know  that  at  some 
time  between  those  times  it  must  have  had  a temperature 
of  15°. 

As  a corollary  of  this  law  we  may  note,  that  if  any  pro- 
perty of  a natural  body  has  at  one  time  a ( + ) value  and  at 
another  time  a (— ) value,  then  at  some  instant  between  those 
times  its  value  must  have  been  zero. 

For  instance,  if  a body  is  at  one  time  above  the  ground 
and  at  another  below  it,  then  at  some  instant  between  those 
times  it  must  have  been  at  the  ground  level. 

A careful  remembrance  of  this  law  wili  save  the  electrical 
inventor  from  many  mistakes  and  difficulties. 

For  instance,  we  must  remember  that  if  the  N.  pole  of  a 
core  changes  to  a S.  pole,  then  that  at  some  period  between 
the  times  when  the  pole  hadN.  and  S.  polarity,  it  must  have 
been  completely  demagnetized.  Again,  if  a current  is 
reversed,  it  must  cease  altogether  between  the  times  of  its 
flowing  in  one  and  the  other  direction. 

Electro-magnetic  Induction. 

If  a wire  be  moved  through  a magnetic  field,  an  electro- 
motive force  will  be  produced  between  its  ends  which  will 
be  simply  proportional  to  the  number  of  lines  of  force 
cut  by  it  per  second;  lines  cut  in  one  direction  being 
reckoned  +,  those  cut  in  the  other  direction  being  reckoned 


1 1 2 Electric  Lighting . 

— , the  sign  being  also  reversed  if  the  polarity  of  the  field 
is  reversed. 

The  number  of  lines  of  force  cut  per  second  depends. 

First  (in  a uniform  field),  on  the  length  measured  in  a 
straight  line  from  one  end  of  the  moving 
wire  to  the  other ; i.e.  if  A B C.,  fig.  57 
be  the  wire,  it  depends  on  the  length 
ADC. 

Second,  on  the  angle  which  the  direction 
of  motion  makes  with  the  lines  of  force.  If 
the  wire  moves  along  the  lines  of  force,  it 
will  cut  none  of  them ; if  at  right  angles  to  them,  it  will  cut 
the  maximum  number.  The  number  cut  is  directly  pro- 
portional to  the  sine  of  the  angle  which  the  direction  of 
motion  makes  with  the  lines  of  force. 

Third,  on  the  number  of  lines  of  force  which  pass  through 
each  unit  of  area  of  the  region  across  which  the  wire  moves, 
i.e.  on  the  strength  of  the  magnetic  field. 

Fourth,  on  the  velocity  of  motion. 

If  the  ends  of  the  wire  are  connected  by  another  wire  not 
in  motion,  a current  will  flow  through  the  wire,  whose 
strength  depends  on  the  electro-motive  force  produced  and 
on  the  resistance  of  the  circuit. 

For  instance,  the  circuit  may  be  completed  by  means  of 
two  fixed  rails,  on  which  the  moving  wire  slides,  the  rails 
being  connected  at  one  end. 

If  in  a uniform  field  the  ends  of  the  wire  are  connected 
by  means  of  a second  wire,  which  also  moves  across  the 
lines  of  force  (fig.  58),  no  current  will  be  produced.  The 

electro-motive  forces  in  the  two 
halves  of  the  ring  formed  by  the 
two  wires  will  be  in  the  same 
absolute  direction  in  space,  and 
therefore  in  opposite  directions  in 
the  ring.  In  fig.  58  we  suppose 
the  lines  of  force  to  be  perpendi- 
cular to  the  paper,  and  to  be  re- 
presented by  the  dots.  We  sup- 
pose the  circuit  to  consist  of  a 




Fig.  68. 


Fig.  57. 


Theory  of  Elective  Generators . 1 13 

curved  wire  whose  ends  are  connected  by  a zigzag  one, 
and  that  it  moves  in  the  direction  of  the  large  arrow.  We 
see  that  each  half  of  the  circuit  cuts  the  same  number  of 
lines  of  force,  and  the  electro- motive  forces  are  both  of  the 
same  magnitude,  and  are  in  opposite  directions  in  the  ring, 
as  represented  by  the  small  arrows;  and  therefore  there  will 
be  no  current. 

The  moving  of  a wire  across  the  lines  of  terrestrial  mag- 
netic force  will  produce  an  electro-motive  force  between  its 
ends.  A large  portion  of  the  earth’s  magnetic  force  is 
vertical;  a horizontal  wire  moved  parallel  to  itself  will 
therefore  cut  terrestrial  magnetic  lines.  Let  us  suppose  the 
rails  of  a railway  to  be  insulated  from  each  other,  but  con- 
nected at  one  end  through  a galvanometer.  The  wheels 
and  axle  of  a railway  carriage  would  complete  the  circuit. 
As  the  carriage  moves,  an  electro -motive  force  will  be  pro- 
duced between  the  ends  of  the  axle,  which  will  produce  a 
current  through  the  galvanometer.  If  there  are  several 
axles  they  will  all  act  in  the  same  direction,  like  batteries  in 
parallel  circuit. 

If,  instead  of  being  connected  to  the  rails,  the  gal- 
vanometer, were  connected  to  the  ends  of  the  axle,  and 
carried  in  the  carriage,  no  current  would  be  produced,  the 
reason  being  that  equal  electro-motive  forces  would  be 
produced  in  the  axle  and  in  the  connecting  wires  of  the 
galvanometer.  These  forces  would  be  in  the  same  abso- 
lute direction,  and  therefore  would  be  opposed  to  each 
other  in  the  circuit. 

Theory  op  Electric  Generators. 

All  electric  generators  consist  of  machines  for  moving 
wires  past  magnets,  or  magnets  past  wires,  the  connections 
being  so  arranged  that  the  electro-motive  forces  generated 
may  produce  currents. 

If  our  moving  circuit  consists  of  a ring  which  is  suffi- 
ciently large  in  comparison  with  the  field,  we  can  cause  one 
side  of  it  to  move  over  the  N.  pole  of  a magnet  while  the 
other  side  is  moving  over  the  S.  pole,  and  the  electro-motive 
forces  produced  in  the  two  halves  will  then  be  in  opposite 

1 


1 14  Electric  Lighting. 

directions  in  space,  and  therefore  in  the  same  directions  in 
the  ring,  and  currents  will  circulate. 

Let  us  suppose  our  moving  circuit  to  consist  of  the  two 
axles  of  a four-wheeled  railway  carriage  (fig.  59),  connected 


u 

D 

® 

© 

Fig.  59. 

by  wires  running  along  the  sides  of  the  carriage,  and  passing 
through  a galvanometer  carried  in  it ; and  suppose  that  in- 
stead of  making  use  of  terrestrial  magnetism,  we  bury  in 
the  permanent  way  a number  of  powerful  magnets  (fig.  59) 
of  alternate  polarity,  the  distance  between  the  poles 
being  equal  to  the  distance  between  the  axles.  We  see 
that  the  electro-motive  forces  produced  in  the  two  axles  will 
be  in  opposite  directions,  and  therefore  a current  will  circu- 
late until  the  axles  arrive  at  the  neutral  point  between  two 
poles.  The  current  will  then  diminish  to  nothing,  and  then 
gradually  increase  again ; but  as  the  field  now  being  passed 
through  by  each  axle  is  of  opposite  polarity  to  what  it  was 
before,  the  current  will  be  in  the  opposite  direction.  And 
thus  as  the  carriage  moves  on,  currents  will  be  produced 
which  will  be  reversed  in  direction  each  time  the  centre  of 
the  carriage  passes  a pole. 

The  principle  of  this  arrangement  is  the  basis  of  all 

alternating- current  machines. 

We  see,  therefore,  that  the  only  way  in  which  currents  can 
be  induced  in  a closed  ring  or  coil  of  wire,  is  by  the  approach 
to  or  recession  of  the  coil  from  a pole. 

A motion  through  a uniform  field  produces  equal  and  opposite 
electro-motive  forces  in  the  two  sides  of  the  coil  or  ring, 
which  forces  neutralize  each  other. 

If  the  floor  of  the  carriage  in  fig.  59  had  consisted  of  a 
thick  iron  plate,  the  electro-motive  force  produced  would 
have  been  greater,  for  the  lines  of  force  would  have  been 
nearly  vertical,  as  in  fig.  60,  instead  of  partly  horizontal,  as 
in  fig.  61. 


Theory  of  Electric  Generators.  1 1 5 

In  practice,  the  only  way  in  which  wires  can  be  moved 
rapidly  past  magnets  is  by  attaching  them  to  the  periphery 
of  a revolving  wheel  round  which  stationary  magnets  are 


arranged,  so  that  the  wires  pass  the  same  magnets  again  and 
again.* 

Another  way  of  enabling  the  induced  electro-motive  forces 
to  produce  currents,  is  that  invented  by  Professor  Paccinotti 
in  1863,  and  known  as  the  “ Paccinotti  or  Gramme  ring/’ 
By  this  apparatus  the  currents  are  produced  continuously  in 
one  direction.  Its  principle  is  the  basis  of  all  continuous- 
current  machines.  We  shall  fully  describe  it  in  Chapter  XIII. 


Direction  of  the  Induced  Currents — Lenz’s  Law. 

In  1834  f Lenz  enunciated  the  following  remarkable 
law  : — 

Whenever  a current  is  induced  in  a circuit  by  the  relative 
motion  of  the  circuit  and  of  a magnet , or  of  another  circuit 
carrying  a current , the  direction  of  the  induced  current  is  such 
that  by  its  attraction  or  repulsion  on  the  inducing  magnet  or 
circuit  it  opposes  the  motion. 

We  see  that  if  this  were  not  so  we  should  have  a “ per- 
petual motion,”  as  the  induced  current  might  produce  the 
motion  which  itself  produced  the  induced  current. 


Induction  by  Variation  of  Current  in  one  of  two 
Stationary  Circuits. 

If  an  electro-magnet,  or  a circuit  (which  may  be  regarded 
as  an  electro-magnet  without  an  iron  core)  be  placed  near  a 

* Or  the  magnets  may  be  attached  to  the  revolving-wheel  and  moved 
past  fixed  wires. 

f Pogg.,  Ann.  xxxi.  483  (1834), 


1 1 6 Electric  L ighting. 

coil  of  wire,  and  the  current  in  tbe  electro- magnet  be  made 
to  vary,  currents  will  be  induced  in  the  circuit  as  long  as  the 
variation  continues. 

The  direction  of  the  current  produced  by  increasing  mag- 
netism is  the  same  as  that  produced  by  an  approaching  pole, 
that  of  the  current  produced  by  decreasing  magnetism  is  the 
same  as  that  produced  by  a receding  pole.  An  increasing 
current  in  the  electro-magnet  induces  a current  in  a direction 
opposite  to  its  own  ; a decreasing  current,  one  in  the  same 
direction. 

Ikon  Coke. 

An  iron  core  may  be  placed  in  the  circuit  in  which  a cur- 
rent is  to  be  produced.  This  generally  increases  the  effect 
as  it  strengthens  and  concentrates  the  lines  of  force. 

As  the  directions  of  the  induced  current  have  a constant 
relation  with  the  changes  of  polarity  of  the  core,  we  may, 
if  we  please,  study  the  latter  instead  of  the  former.  The 
actions  are  somewhat  easier  to  follow. 


Effect  of  the  Iron  Core  on  the  Coil  surrounding  it. 

If  the  magnetism  of  the  iron  core  is  altered,  as,  for  instance, 
by  moving  a magnet  to  and  from  it,  currents  will  flow  in  the 
coil  as  long  as  the  change  continues. 

While  the  magnetism  of  the  core  is  increasing,  the  direc- 
tion of  the  induced  current  will  be  such  that  it  will  tend  to 
make  the  iron  core  a magnet  having  opposite  polarity  to 
that  actually  caused  by  the  induction  of  the  neighbouring 
magnet. 

While  the  magnetism  is  decreasing , the  direction  of  the 
induced  current  will  be  such  that  it  will  cend  to  make  the 
iron  core  a magnet  having  the  same  polarity  as  that  actually 
caused  by  the  induction  of  the  neighbouring  magnet. 

Moving  Magnet. 

The  motion  of  a magnet  past  a coil  will  be' fully  discussed 
in  the  next  chapter. 


Electro - M agnets. 


ii  7 


Construction  of  Electro-Magnets  and  Measurement  of 
Magnetic  Fields. 

General  Rule. 

When  we  have  a dynamo  machine  with  magnets  of  a 
certain  size,,  and  wish  to  construct  another  with  magnets  of 
a different  size,  it  is  extremely  important  that  we  should  be 
able  to  determine  what  difference  in  the  strength  of  the 
field  will  be  produced  by  the  proposed  change  of  size,  or 
rather  what  change  in  size  must  be  made  to  produce  a 
desired  change  in  the  strength  of  field.  It  is  impossible  to 
calculate  this  accurately  beforehand,  but  by  the  method  I 
am  about  to  describe  the  proper  size  of  the  magnets  may  be 
approximately  calculated,  and  then  the  field  produced  may 
be  accurately  measured. 

With  magnets  of  the  same  general  shape  and  proportions  not 
magnetized  to  saturation , expending  the  same  horse-power  of 
magnetizing  electricity  in  every  pound  weight  or  in  every  cubic 
inch  of  copper  in  the  helix , the  magnetic  field  produced  is 
approximately  proportional  to  the  total  weight  of  the  magnets. 

Saturation. 

When  a feeble  current  is  sent  round  the  coils  of  an 
electro-magnet,  the  magnetism  is  proportional  to  the  current 
as  the  latter  increases  up  to  a certain  point. 

After  a certain  point  the  magnetism  increases  less  rapidly 
than  the  current,  the  relative  rate  of  increase  becoming  less 
and  less,  until  at  last  a point  is  reached  where  further 
increase  of  current  produces  no  increase  of  magnetism. 
When  this  point  is  reached  the  magnet  is  said  to  be  saturated. 
In  magnets  intended  for  use  in  dynamo  machines,  the  satu- 
rating point  should  not  be  approached.  The  maximum 
current  that  should  be  used  is  a current  but  little  in 
excess  of  that  for  which  the  magnetism  is  nearly  propor- 
tional to  the  current. 

Measurement  of  Magnetic  Field. 

The  following  instrument  (Plate  XY.)  has  been  designed 
by  me  for  measuring  the  intensity  of  a magnetic  field  at 


1 1 S Electric  Lighting. 

any  point.  It  is  a modification  of  one  invented  by  tbe  late 
M.  Verdet.* 

Tbe  principle  on  wbicb  tbe  apparatus  works  is  that,  if  a 
small  coil  of  wire,  placed  in  a magnetic  field,  moves  rapidly 
for  a quarter- turn  round  an  axis  at  right  angles  to  tbe  axis 
of  the  coil,  then  tbe  total  quantity  of  electricity  generated 
is  simply  proportional  to  tbe  strength  of  tbe  magnetic  field 
at  tbe  coil.f  When  tbe  motion  is  rapid,  tbe  quantity  of 
electricity  generated  is  proportional  to  tbe  sine  of  half  tbe 
angle  through  wbicb  tbe  needle  of  a galvanometer  connected 
to  tbe  moving  coil  will  swing. 

By  using  a reflecting  galvanometer  and  diminishing  its 
sensibility  by  shunts, J the  angle  of  swing  can  be  made  very 
small,  and  then  the  strength  of  the  magnetic  field  will  be 
simply  proportional  to  the  number  of  divisions  of  the  scale 
passed  over  by  the  first  swing  of  the  light-spot. 

Tbe  practical  form  of  the  instrument  is  shown  in  Plate  XV. 

The  little  coil  consists  of  an  ebonite  bobbin  wound  with 
fine  wire,  and  is  held  in  a light  brass  frame  pivotted  on  an 
axis.  Two  stops  limit  its  motion,  so  tbat  it  can  turn  through 
90°.  It  is  forced  against  one  of  tbe  stops  by  means  of  a 
spring.  By  means  of  tbe  handle  it  can  be  turned  till  it  is 
forced  against  tbe  other  stop.  On  tbe  handle  being  released, 
tbe  bobbin  turns  instantly  through  90°,  under  tbe  action  of 
tbe  spring.  Tbe  ends  of  tbe  bobbin  are  connected  to  tbe 
poles  of  tbe  galvanometer  by  means  of  two  thin  wires  twisted 
together,  so  tbat  accidental  motions  of  tbe  connecting  wire 
through  tbe  magnetic  field  may  not  affect  tbe  galvanometer. 

Tbe  stand  of  tbe  instrument  is  so  graduated  tbat  tbe 
exact  position  of  tbe  centre  of  tbe  bobbin  in  tbe  magnetic 
field  can  be  noted,  and  so  a map  of  tbe  latter  can  be  con- 
structed with  tbe  intensity  at  each  point. 

# See  “ (Euvres  de  Verdet,”  tome  i.  notes  et  memoires,  p.  128 ; or  my 
“ Electricity,’’  2nd  ed.  vol.  ii.  p.  253. 

f Tbe  current  is  of  varying  intensity  and  may  be  represented  at 
each  instant  by  dC,  where  C is  tbe  current  in  amperes.  The  total  quantity 
Q of  electricity  generated  in  a time  to  where  Q is  measured  in  coulombs, 
and  t is  the  time  occupied  by  the  bobbin  in  turning  90°,  is 


J The  shunt  used  must  not  be  varied  during  a set  of  experiments. 


Plate  XV. — Gordon’s  magnetic  eield  measuker. 


Gordon's  Magnetic  Field  Measu7rer.  1 1 9 

Lines  are  engraved  on  the  base  from  which  the  horizontal 
distances  in  two  directions  (at  right  angles  to  each  other) 
from  the  centre  of  the  magnet  can  be  measured  by 
the  rule  shown  lying  on  the  base.,  and  divisions  in  the 
vertical  stem  give  the  height  above  the  face  of  the  magnet. 
A gauge  enables  the  bobbin  to  be  set  vertically  over  any 
point  on  the  surface  of  the  pole  plate.  With  magnets  such 
as  are  used  in  my  alternating-current  machines,  the  centre 
of  the  face  of  the  pole  plate  is  taken  as  “ origin,”  distances 
measured  along  the  circumference  of  the  magnet- wheel  are 
called  x,  those  along  the  radius  are  called  y , and  those 
perpendicular  to  the  face  are  called  z. 

Thus,  such  a memorandum  as 

x = 3",  y — 0,  z = 4",  d = 270,  C = 24  amps., 

would  mean  that  at  a point  on  the  circumferential  line 
3 inches  to  one  side  of  the  centre  of  the  magnet  and  4 inches 
from  its  face,  the  magnetic  field  was  such  as  to  cause  the 
light-spot  to  swing  over  270  divisions  of  the  scale  when  the 
magnetizing  current  was  24  amperes. 

To  determine  when  saturation  commences,  an  ammeter  is 
put  in  circuit  with  the  magnet,  and  the  current  being 
increased  2 or  3 amperes  at  a time,  the  magnetic  field  due  to 
each  current  is  measured. 

The  ratio  of  magnetic  field  to  current  is  then  taken  for  each 
experiment,  and  when  it  begins  to  seriously  diminish  with 
increased  current,  we  know  we  are  approaching  the  satu- 
ration point  of  the  magnet  under  examination. 

For  instance,  let  us  suppose  that  with  a particular  magnet 
the  following  results  are  obtained  : C as  before  representing 
the  exciting  current,  d the  strength  of  the  field. 


C 

d 

d 

C 

amps. 

12 

240 

20 

14 

280 

20 

16 

310 

19*3 

18 

340 

18-8 

20 

345 

j 172 

22 

1 

346 

1 

1 15-7 

1 

120 


Electric  Lighting . 

We  see  that  after  about  18  or  20  amperes  we  are 
not  getting  an  increase  of  magnetism  sufficient  to  pay 
the  cost  of  the  extra  amount  of  current  used,  as  we  must 
remember  that  the  H.P.  expended  in  sending  a current 
through  the  magnet-wire  increases  as  the  square  of  that 
current. 

When  used  to  compare  the  relative  strengths  of  the  fields 
produced  by  these  magnets  when  the  same  horse-power  of 
electricity  is  being  expended  per  pound  of  copper,  we 
proceed  as  follows  : — 

One  magnet,  which  is  our  standard,  is  made  precisely 
similar  to  one  in  some  machine  whose  power  is  well  known, 
the  other  is  made  of  the  proposed  new  form.  Both  are 
wound  with  wire  of  the  same  gauge  and  are  connected  in 
series,  so  that  the  same  current  goes  through  both.  An 
ammeter  in  series  with  the  magnets  enables  the  current  to 
be  kept  constant,  and  alternate  observations  are  taken  of 
the  two  fields. 

To  facilitate  the  rapid  placing  of  the  induction  instrument 
in  position,  the  pole  plates  may  be  advantageously  let  flush 
into  wooden  boards  on  which  “ latitude  and  longitude 39 
lines  are  marked. 

An  iron  core  similar  to  the  core  of  the  proposed 
armature  coil  may  be,  if  desired,  fixed  in  position  over  the 
pole,  and  the  disturbance  of  field  produced  by  it  can  then 
be  noted. 


Apparatus  por  use  in  confined  spaces. 

When  it  is  desired  to  measure  the  number  of  lines  of 
force  passing  across  any  very  narrow  space,  as,  for  instance, 
the  number  passing  from  a magnet  pole  to  an  armature 
core,  the  following  modification  of  the  instrument  may 
be  used.  The  coil  may  be  wound  in  the  form  of  a flat  disc 
only  one  wire  thick,  and  may  be  gummed  between  thin 
strips  of  card-board.  This  can  be  slid  between  the  magnet 
pole  and  armature  coil,  and  if  jerked  suddenly  out,  currents 
will  be  induced  in  it,  whose  total  value  is  determined  by 
the  swing  of  the  galvanometer  needle  exactly  as  described 
above. 


Mutual  Induction. 


I 2 I 


Comparative  Measures  of  the  Coefficients  of  Mutual  In- 
ductions OF  VARIOUS  SHAPED  CoiLS  AND  ELECTRO-MAGNETS. 

Let  us  suppose  tliat  we  have  a machine  in  which  the 
magnets  and  coils  have  a certain  shape,  and  that  we  wish  to 
see  whether  some  other  shape  is  better  or  worse.  The  only 
way  in  which  this  can  be  determined  accurately  is  by  con- 
structing a machine  on  the  new  plan,  and  trying  it. 

The  following  method#  will,  however,  give  results  suffi- 
ciently accurate  to  be  a very  useful  guide  in  devising  the 
new  machine,  and  involves  only  the  comparatively  trifling 
expense  of  making  one  magnet  and  one  coil.  The  old  and 
new  coils  are  each  placed  in  the  field  of  their  magnets.  In 


Fig.  62. 


the  case  of  the  old  coil  this  may  be  done  without  taking  the 
machine  to  pieces.  The  electro-magnets  are  connected  in 
series,  so  that  the  same  battery  or  machine  current  can  be 
sent  through  both.  On  this  current  being  made  and  broken, 
transient  currents  will  be  induced  in  the  two  coils  respec- 
tively. The  coils  are  to  be  connected  to  each  other  “ in 
quantity,”  and  their  joined  ends  to  the  two  poles  of  a 
galvanometer,  as  in  fig.  62,  the  connections  being  such 
that  the  currents  sent  through  the  galvanometer  by  the 
two  coils  respectively  are  in  opposite  directions. 

On  making  and  breaking  contact,  it  will  probably  be 
found  that  the  action  of  one  of  the  coils  is  greater  than  that 

* This  paragraph  is  an  attempt  to  explain  in  unmathematical  language 
the  method  given  in  Maxwell’s  “Electricity,’’  § 755,  vol.  ii.  p.  364. 


122  Electric  Lighting, 

of  the  other,  and  consequently  the  needle  is  deflected.  A re- 
sistance box  R being  introduced  into  the  stronger  circuit,  the 
resistance  is  increased  until  the  deflection  is  reduced  to  zero. 

The  currents  being  then  equal,  the  E.M.F.s  induced  in 
the  two  portions  of  the  circuit  are  directly  proportional  to 
the  resistances ; or  if  rx  is  the  resistance  of  the  left-hand 
coil,  M:  the  coefficient  of  mutual  induction  between  it  and 
its  magnet,  and  r2,  M2  the  corresponding  quantities  for  the 
right-hand  coil,  we  shall  have, — 


or,— 


M,  : M2  * * rx  : r2  -f-  R, 

M2 H-  R 


(45) 


The  relative  inductions  in  different  parts  of  the  field  can 
be  of  course  determined  by  placing  the  coils  in  different 
symmetrical  portions  with  regard  to  the  magnets,  i.e.  by 
moving  them  both  through  equal  fractions  of  the  total 
distance  traversed  in  a complete  phase. 


Self-Induction. 

When  a varying  current,  whether  alternating  or  merely 
increasing  and  diminishing,  is  sent  through  a wire,  it  acts 
inductively  on  all  wires  in  its  neighbourhood,  whether  those 
wires  belong  to  other  circuits,  or  are  other  convolutions  of 
the  same  coil. 

When  insulated  wire  is  wound  into  a coil,  and  a varying 
current  sent  through  it,  each  convolution  acts  inductively  on 
all  the  other  convolutions.  This  action  is  called  self-induction. 

While  the  current  in  a coil  is  increasing  the  mutual  action 
of  the  wires  delays  the  increase,  because  it  causes  an  E.M.F. 
in  the  direction  opposed  to  that  generating  the  current. 

While  the  current  is  decreasing,  the  mutual  action  of  the 
wires  delays  the  decrease,  because  it  causes  an  E.M.F.  in 
the  same  direction  as  that  generating  the  current. 

We  see  that  one  effect  of  the  self-induction  is  to  retard 
the  phase  of  the  current,  i.e.  to  cause  the  alternations  to 
take  place  a little  later  than  they  would  otherwise  have 
done.  This  retardation  would  not  be  noticed  in  any  prac- 
tical applications,  so  we  need  not  concern  ourselves  with  it. 


Self-Induction . 


123 


The  second  effect  of  self-induction  is  to  diminish  the 
current. 

It  is  found  experimentally,  and  can  be  proved  mathe- 
matically, that  if  a coil  of  wire  forms  part  of  a circuit, 
and  an  alternating  E.M.F.  sends  a current  through  it,  that 
the  current  will  be  much  less  than  it  would  have  been  had  the 
same  resistance  been  interposed  in  the  form  of  a straight  wire. 

Further , the  proportional  diminution  becomes  greater  as  the 
current  is  increased  by  the  reduction  of  the  resistance ; and 
finally,  for  a given  F.M.F.,  a given  rate  of  alternation,  and  a 
coil  of  given  shape,  a limit  is  reached  beyond  which  even 
reducing  the  resistance  to  zero  does  not  increase  the  current. 

Tor  example:  Suppose  the  coil  to  form  one  coil  of  a 
dynamo  machine,*  and  let  the  rest  of  the  circuit  be 
incandescent  lamps  arranged  in  quantity,  and  let  the 
resistance  of  the  coil  be  very  small  compared  to  that  even 
of  a considerable  number  of  lamps,  then  if  there  were  no 
self-induction  the  current  should  proportion  itself  to  the 
number  of  lamps  inserted,  as  explained  on  page  26. 

The  effect  of  self-induction  is,  that,  after  a certain  number 
of  lamps  are  inserted,  a limit  is  reached  when  diminishing 
the  resistance  by  the  insertion  of  more  lamps  does  not  pro- 
portionately increase  the  current ; and  that  limit  represents 
the  number  of  those  lamps  which  can  be  maintained  on  that 
circuit. 

The  mathematical  proof  of  the  above  propositions  is  of  a 
complex  nature,  but  the  geometrical  diagram  (fig.  63)  is 
useful  to  illustrate  the  fact  that  self-induction  diminishes 
the  current. 


c 


In  fig.  63  let  horizontal 
distance  represent  time, 
and  let  vertical  height 
above  the  line  A Bf  re- 
present strength  of  cur- 
rent. Let  the  points  A 
and  B represent  the  in- 


* It  must  be  remembered  that  all  dynamo-machines  produce  alter- 
nating currents,  though  in  some  machines  the  currents  are  made  direct 
after  they  leave  the  coils  and  before  they  leave  the  machine. 


124  Electric  L ighting. 

stants  when  a N.  and  a S.  pole  respectively  pass  the  core 
of  the  coil. 

Now,  if  there  were  no  self-induction  the  rise  and  fall  of 
the  current  would  be  represented  by  the  curve  A C B.  The 
current  would  begin  to  increase  from  zero  at  the  instant 
A as  the  N.  pole  passed  the  core,  and  would  return  to  zero 
at  the  instant  B as  the  S.  pole  passed.  The  height  D C 
represents  the  maximum  current ; the  steepness  of  the  slope 
A C (i.e.  the  angle  D A C)  represents  the  rate  of  in- 
crease ; that  of  the  slope  B C (i.e.  the  angle  DBC)  the 
rate  of  decrease. 

Now  one  effect  of  self-induction  is  to  retard  the  phase, 
i.e.  to  shift  the  zero  points  A.  B to  the  positions  A'  B7.  It 
does  not,  however,  alter  the  length  of  the  phase,  and  there- 
fore the  length  A'  B7  remains  equal  to  the  length  A B. 

But  both  the  rate  of  increase  and  the  rate  of  decrease  of 
the  current  are  reduced  by  self-induction,  and  hence  the 
slopes  of  the  sides  of  the  triangle  are  reduced,  and  are  now 
represented  by  the  sides  A7  C7  and  B7  C'. 

But  if  there  are  two  triangles  on  equal  bases,  the  one  with 
the  smallest  slope  of  sides  must  be  the  lowest,  or  must  be 
contained  between  narrower  parallels  than  the  other ; and  we 
see  that  the  height  D7  C7  of  the  triangle  A'  B7  C7  is  less  than 
that  of  the  first  triangle. 

Now  the  total  quantity  of  electricity  which  has  passed 
during  the  phase,  or  the  total  integral  value  of  the  current, 
is  represented  by  the  total  area  of  the  curve ; i.e.  the  area 
of  the  triangle  ABC  represents  the  total  quantity  of  elec- 
tricity which  would  have  passed  if  there  had  been  no  self- 
induction,  and  the  area  of  the  triangle  A'  B7  C7  represents 
the  total  quantity  which  has  actually  passed. 

It  is  easy  to  see  that  the  triangle  A'  B7  C7  is  the  smaller  of  the 
two ; for  we  know  from  Euclid  that  triangles  on  equal  bases 
and  between  the  same  parallels  are  equal,  and  hence  of  two 
triangles  on  equal  bases  and  between  different  parallels,  the 
one  between  the  narrowest  parallels  must  be  the  smallest. 

We  have  hitherto  assumed  that  the  retardations  of  both 
the  increase  and  decrease  are  equal;  but  if  they  were  not, 
but  the  curve  had  the  form  A7  C7/  B7,  the  reasoning  would 


Self-Induction.  125 

not  be  affected,  for  the  triangles  A'  C1  B'  and  A'  C"  B'  are 
on  the  same  base  and  between  the  same  parallels,  and  hence 
are  equal. 


Coefficient  of  Self-induction. 

The  amount  of  self-induction  which  takes  place  with  a 
given  E.M.F.,  a given  rate  of  alternation,  and  a given  re- 
sistance, depends  solely  on  the  shape  of  the  coil.  For  each 
coil  there  exists  a quantity  called  the  coefficient  of  self-in- 
duction. For  a few  simple  forms  of  coils  without  iron  cores, 
mathematicians  are  able  to  calculate  this  quantity.  I am  not 
aware,  however,  that  they  have  succeeded  in  doing  it  for  any 
of  the  oval,  wedge-shaped,  and  ring-shaped  coils  most 
commonly  used  in  machine  construction,  nor  for  any  coils 
having  iron  cores  such  as  are  employed  in  nearly  all  the 
machines  in  use  except  the  alternating  Siemens.  We  can, 
however,  obtain  from  the  results  of  mathematical  analysis 
certain  information  as  to  which  form  of  coils  are  better  and 
which  worse  than  certain  other  forms,  it  being  remembered 
that  our  object  in  settling  the  shape  of  our  coils  is  to  make 
the  self-induction  as  small  as  possible  consistently  with  the 
necessity  of  making  the  induction  of  the  magnets  as  large  as 
possible. 

The  circuit  of  minimum  self-induction  is  a wire  doubled 
back  on  itself,  so  that  the  two  equal  currents  flowing  in 
opposite  directions  are  close  together.  This  form  cannot  be 
used  in  machine  construction,  as  opposed  E.M.F.s  would  be 
induced  in  the  two  halves. 

All  resistance  coils  are  wound  in  this  way,  i.e.  the  wire 
is  doubled  in  the  middle,  the  two  ends  are  attached  to 
the  terminals,  and  the  doubled  wire  wound  on  the  reel. 
They  have  thus  neither  self-induction  nor  mutual-induction. 

The  next  best  form  is  a circuit  consisting  of  a single  thin 
wire.  The  thinness  of  the  wire  which  can  be  used  is  in  a 
machine  limited  by  its  resistance.  When  a single  thick  wire 
is  used,  the  self-induction  begins  to  increase,  owing  to  the 
mutual  action  of  the  currents  in  different  parts  of  the  section 
on  each  other.  In  fact  a thick  wire  acts  like  a bundle  of 
thin  ones. 


126 


Electric  Lighting. 

With  a coil  of  wire  the  self-induction  is  again  increased. 
I do  not  know  of  any  experiments  or  results  of  mathematical 
analysis  which  give  us  any  information  as  to  the  best  shape 
of  coil  which  can  be  got  into  a magnetic  field  of  a given 
strength,  and  whether  it  should  be  extended  laterally  or 
longitudinally,  i.e.  whether  (the  lines  of  force  being  for  in- 
stance along  the  axis  of  the  coil)  the  latter  should  be  a rod 
or  a disc. 

Professor  Clerk  Maxwell  has,  however,  calculated*  the 
form  which  a coil  without  an  iron  core  should  have  so  as  to 
have  the  maximum  coefficient  of  self-induction , i.e.  to  be  the 
worst  possible  coil  for  use  in  a machine.  To  obtain  this 
result  Professor  Maxwell  finds  that — * 

If  the  transverse  section  of  the  coil  is  circular,  the  mean 
diameter  of  the  coil  should  be  3.22  (fig.  64)  times  the  dia- 
meter of  the  circular  wire  channel. 


Fig.  61.  Fig.  65. 


If  the  wire  is  wound  in  a square  groove,  i.e.  if  the  groove 
has  a square  transverse  section,  the  mean  diameter  of  the 
coil  should  be  3.7  times  the  side  of  the  wire  channel. 
(Pig.  65.) 

We  see,  therefore,  that  in  constructing  a machine  we 
must  make  the  shape  of  the  coils  differ  as  widely  as  possible 
from  the  above  proportions. 

When  an  iron  core  is  used,  the  coefficient  of  self-induction 
is  greatly  increased.  It  must,  however,  be  remembered  that 
the  mutual  induction  between  the  coil  and  the  magnets  is 
increased  at  the  same  time.  Thus  the  introduction  of  an 
iron  core  increases  the  current  by  increasing  the  action  be- 

* Maxwell's  “ Electricity,5'  § 706,  vol.  ii.  p.  316. 


Effect  of  1 r on  Core  on  Self-Induction . 1 2 7 

tween  coils  and  magnets,  and  decreases  it  by  increasing  the 
self-induction.  Whether  the  net  result  is  an  increase  or 
decrease  of  current,  is  a point  on  which  there  has  been  a 
great  deal  of  controversy. 

My  own  opinion  is  strongly  in  favour  of  iron  cores,  and 
I use  them  freely  in  my  own  machines,  as  do  Edison, 
Crompton,  and  others ; Messrs.  ’ Siemens  Brothers,  and 
Messrs.  Ferranti  make  their  machines  without  them. 

In  support  of  my  own  opinion,  I may  quote  an  ex- 
periment made  on  the  Lachaussee-Lambotte  machine 
during  the  Paris  Exhibition  of  1881.  The  Lachaussee- 
Lambotte  machine  is  one  with  a number  of  sepa- 
rate coils.  Two  coils  were  constructed,  one  with  an  iron 
core  and  one  without,  all  other  conditions  being  exactly 
identical.  It  was  found  that  the  output  of  the  coil  with  the 
iron  core  was  “ notably  superior ” to  that  of  the  other.* 

Comparison  op  the  Coeppicients  op  Self-induction  of  Two 

Coils. 

If  we  are  doubtful  which  of  two  coils  is  the  best  to  use 
in  a machine,  we  may  compare  their  coefficients  of  self- 
induction  by  means  of  the  following  rule  which  is  given 
by  Maxwell : — 

Insert  the  two  coils  into  two  adjacent  branches  of  a 
Wheatstone's  bridge  (fig.  66) . 


# Guerout,  “La  Lumiere  Electrique5’  (Journal),  tom.  iv.,  p.  389^ 
Sept,  24,  1881. 


128 


Electric  Lighting. 


Let  the  coefficients  of  self-induction  of  the  two  coils  be  L 
and  N respectively.  Let  there  be  resistances  a , b in  the 
same  sides  of  the  bridge  as  the  coils  L and  N respectively. 
Let  the  total  resistance  of  the  coil  L,  and  the  resistance  a be 
B,  and  that  of  N and  b be  r. 

Then  in  order  that  there  shall  be  no  permanent  current 
through  the  galvanometer,  we  must  have,  as  before,  the 
products  of  opposite  sides  of  the  bridge  equal,  or, — 

Sr  = sU (42)  * 

The  permanent  current  depends  only  on  the  resistances ; 
the  transient  current,  i.e.  the  sudden  moving  of  the  needle 
on  making  and  breaking  contact,  depends  on  the  ratio  of 
the  coefficients  of  self-induction  of  the  two  coils. 

Professor  Maxwell  has  shown  f that  the  condition  for  no 
transient  current  is, — 

i-? 

Now  we  cannot  tell  whether  there  is  or  is  not  a transient 
current,  until  we  have  arranged  that  there  shall  be  no  per- 
manent current.  We  must  therefore  vary  the  resistances 
till  there  is  no  permanent  current,  and  then  see  if  there  is 
a transient  current.  If  there  is,  we  must  alter  one  of  the 
resistances  a,  b,  and  at  the  same  time  alter  the  resistance 
s or  S,  till  there  is  again  no  permanent  current.  We  must 
then  again  try  if  there  is  a transient  current.  We  must  go 
on  repeating  the  adjustments  till  there  is  no  deflection  of  the 
needle,  either  permanent  or  transient,  on  making  or  breaking 
contact.  When  this  is  the  case,  equation  (46)  gives  the 
ratio  of  the  coefficients,  or  we  may  write  it,— 

L _ E 

E r 


(47) 


Practical  Method. 

The  above  method  is  a troublesome  one  to  use,  and  does 


* Page  53. 

f The  condition  of  no  galvanometer  current  is,-  - 

(^+lS)^=sK~+nS 

where  x and  y are  the  currents  in  the  two  branches  respectively. — 
Maxwell’s  “ Electricity,”  § 757,  vol.  ii.  p.  367. 


Measurement  of  Self-Induction,  129 

not  give  its  result  in  a very  useful  form.  The  following 
method  may  be  used  in  practical  work  : — 

To  find  how  much  a current  of  given  E.M.F.,  and  of  a, 
given  rate  of  alternation,  will  he  diminished  by  the  self- 
induction  of  any  coil. 

Take  the  resistance  of  the  coil  in  the  ordinary  way,  then 
the  amount  of  direct  current  which  would  pass  through  it 
with  the  given  E.M.F.  is  given  by  the  formula  (1),  p.  12, — 


Then  by  means  of  an  electro-dynamometer  measure  the 
amount  CA  of  alternating  current  which  will  pass  with  the 
same  mean  E.M.F.  Tben 


PS1  = 100.  C*  . . . (48) 

CD 

is  the  percentage  diminution  of  current  caused  by  the  self- 
induction,  and 

Rsi  = ^ R (49) 

is  the  apparent  resistance  due  to  the  true  resistance  R and 
to  the  self-induction  acting  together. 

The  apparent  resistance  is  also  given  by  the  formula, — 

Rsi  = i (50)* 

These  quantities  PSI,  RSI,  will  be  different  with  different 
E.M.F.s  and  different  numbers  of  alternations. 

For  a given  E.M.F.  and  alternation  rate,  the  ratio  ~ 

CA 

will  be  different  with  different  values  of  R -F  p,  where  p is 
any  other  resistance  that  may  be  in  seines  with  R.  In 
any  given  electrically  lighted  district,  the  E.M.F.  and  the 
alternation  rate  will  be  constant. 

* Compare  (3),  page  12. 


K 


130 


Electric  Lighting . 


CHAPTER  X. 

General  Principles  of  Electric  Generators. 

All  electric  generators  may  be  considered  as  machines 
for  moving  magnets  past  coils  of  wire  (which  coils  may 
or  may  not  have  iron  cores)  or  coils  of  wire  past  magnets. 

The  motion  is  always  circular,  i.e.  the  coils  are  attached 
to  the  rim  of  a wheel,  so  that  as  the  rim  revolves,  they 
pass  the  magnets  again  and  again. 

Efficiency  of  Machines. 

In  a well-constructed  dynamo  machine  of  any  consider- 
able size  the  amount  of  horse-power  wasted  in  mechanical 
friction  is  extremely  small.  Almost  the  whole  horse-power 
expended  is  used  in  producing  electric  currents,  which  in 
their  turn  produce  heat. 

Part  of  the  heat  is  produced  in  the  lamps,  part  in  the 
conducting  wires,  and  part  in  the  coils  of  the  machine 
itself.  The  heat  produced  in  the  lamps  is  useful,  that  pro- 
duced in  the  machine  and  wires  is  useless  and  injurious. 

The  efficiency  of  a machine  is  the  ratio  of  the  useful  to  the 
total  heat  produced , that  isy  of  the  useful  to  the  sum  of  the 
useful  a,nd  useless  heat. 

Our  object  in  constructing  a machine  is  to  make  this 
ratio  as  high  as  possible,  or  in  other  words,  to  make  the 
quantity  of  heat  produced  in  the  machine  and  leading  wires 
as  low  as  possible.  Now  the  relative  amounts  of  heat  pro- 
duced in  different  parts  of  a circuit  all  traversed  by  the 
same  current  are  simply  proportional  to  the  relative  resist- 
ances of  the  different  parts. 

We  can  diminish  the  heating  of  the  leading  wires  by 


Principles  of  Electric  Generators . 1 3 1 

making  them  thicker,  the  limit  being  reached  when  the 
annual  interest  on  the  cost  of  the  wires  becomes  a more 
serious  item  of  expense  than  the  annual  cost  of  the  heat 
wasted. 

We  must  diminish  the  resistance  of  the  wire  of  the 
machine  as  much  as  possible,  without  unduly  diminishing 
the  E.M.F.  We  must  remember  that  the  E.M.F.  depends 
among  other  things  on  the  length  of  wire  on  the  coil,  for 
the  longer  the  wire  the  more  lines  of  force  it  will  cut. 

If  we  make  our  wire  long  and  thin  we  shall  increase  our 
resistance;  if  we  make  it  long  and  thick,  a considerable 
part  of  it  will  be  a long  way  from  the  poles  of  the  magnets. 

We  must  remember,  however,  that  increasing  the  length 
of  wire  is  not  the  only  way  to  increase  the  E.M.F.,  for  the 
E.M.F.  is  also  increased  by  increasing  the  strength  of  the 
magnetic  field,  and  by  increasing  the  velocity  with  which 
the  wires  move  through  it,  or  with  which  it  moves  over  the 
wires. 

The  magnets  may  be  made  very  strong.  The  limit  of 
their  power  is,  if  steel  magnets  are  used,  given  by  the  fact 
that  the  size  of  a steel  magnet  increases  with  its  power, 
and  if  it  is  very  large  its  pole  cannot  all  get  near  the  wire. 

With  electro-magnets  the  size  is  limited  by  the  expense 
of  the  current  required  to  excite  them. 

The  third  way  of  increasing  the  E.M.F.  is  by  increasing 
the  speed  of  the  machine.  The  possible  speed  at  which  a 
machine  may  be  run  is  limited  only  by  the  centrifugal  force 
tending  to  make  it  fly  to  pieces.  The  practical  speed  is 
limited  by  mechanical  and  engineering  considerations  to 
a much  lower  limit  than  would  be  allowed  by  the  centrifugal 
force,  for  high  speed  means  increased  wear  and  tear  of 
machinery  and  greater  liability  to  break  down,  and  it  is 
very  bad  policy  to  diminish  the  cost  of  a machine  per  lamp 
say  twenty  per  cent.,  by  an  increase  of  speed  which  will 
double  or  treble  the  depreciation  rate. 

General  Types. 

An  immense  number  of  generators  have  been  devised,  of 

k 2 


j 32  Electric  Lighting. 

different  forms,  but  they  may  all  be  resolved  into  two 
general  types,  viz. : — 

(1)  The  “ alternating”  type,  where  a number  of  coils  are 
placed  on  the  rim  of  one  wheel,  and  a number  of  magnets 
on  the  rim  of  another  concentric  with  it,  and  one  of  the 
wheels  being  fixed,  the  other  is  caused  to  revolve.  “ Alter- 
nating currents,”  i.e.  currents  alternately  in  opposite 
directions,  are  produced  in  the  coils,  and  are  either  used  as 
alternating  currents,  or  are  converted  into  direct  currents 
by  being  passed  through  a “ commutator  ” before  they  go 
into  the  line. 

(2)  The  “ direct  ” type,  where  the  wire  is  wound  con- 
tinuously round  a ring  of  iron,  the  winding  being  as  if  the 
wire  were  wound  spirally  round  a long  iron  bar,  which  was 
afterwards  bent  so  as  to  form  a ring.  In  this  type  the  ring 
revolves  between  two  (or  sometimes  four  or  six)  magnet- 
poles,  and  although  the  currents  are  alternately  in  opposite 
directions  in  any  portion  of  the  wire,  they  are  always  in  the 
same  direction  in  the  wires  which  are  passing  either  pole. 
These  currents  are  “ collected  ” by  suitable  apparatus  and 
are  passed  as  “ direct  currents  ” into  the  line. 

We  shall  first  discuss  the  alternating  type,  as  it  is  the 
simpler  of  the  two,  and  we  will  consider  the — 

Motion  of  a Magnet  Pole  past  a Coil  with  an  Iron 

Core. 

If  the  circuit  is  broken  so  that  no  current  can  be  induced, 
no  work*  is  done  as  the  magnet  passes  the  core,  for  it  in- 
duces a polarity  opposite  to  its  own,  causing  an  attraction  ; 
and  as  it  approaches  the  core,  the  attraction  helps  the  motion; 
and  as  it  leaves  it,  it  retards  the  motion  ; and  these  two 
opposite  actions  are  equal. 

We  note  that  while  the  magnet  is  approaching  the  core 
the  magnetism  of  the  latter  is  increasing , while  it  leaves  it  is 
decreasing. 

# No  work  is  done  by  an  E.M.F.  when  the  current  cannot  flow. 
Compare  equation  10,  page  16  : — 

H.P.  = When  K is  infinite,  H.P.  is  zero. 

746U 


Motion  of  Magnet  past  Coil.  133 

When  the  circuit  of  the  coil  surrounding  the  core  is  closed, 
currents  flow  such  that,  while  the  magnet  is  approaching  and 
the  magnetism  of  the  core  increasing,  they  tend  to  cause 
a polarity  opposite  to  the  induced  polarity  of  the  core,  and 
the  same  as  the  polarity  of  the  approaching  magnet,  and  hence 
diminish  the  attraction  which  is  helping  the  motion. 

When  the  magnet  has  passed  the  core,  and  the  magnetism 
of  the  latter  is  decreasing,  the  induced  currents  tend  to 
cause  a polarity  the  same  as  that  induced  by  the  magnet, 
and  hence  increase  the  attraction  which  is  retarding  the 
motion  of  the  magnet. 

Work  has  then  to  be  expended  in  moving  the  magnet, 
and  this  work  is  proportional  to  the  sum  of  the  two 
differences  of  attraction  mentioned,  namely,  to  the  sum  of  the 
diminution  of  attraction  when  the  poles  were  approaching, 
and  of  the  increase  of  attraction  when  they  were  re- 
ceding. This  work  is  the  work  expended  in  producing  the 
currents. 

The  above  argument  may  be  more  concisely  expressed 
by  means  of  symbols.*  Let  W be  the  total  work  required 
to  move  the  magnet  from  the  iron  core  to  a distant  point 
when  no  current  is  allowed  to  flow.  Then  — W will  be 
the  work  required  to  move  it  from  a distant  point  to  the 
core.  The  total  work  required  to  move  it  past  the  core,  i.e. 
from  a distant  point  on  one  side  to  a distant  point  on  the 
other,  will  be  the  sum  of  these  two  quantities,  i.e.  will  be 

W - W = 0. 

When  the  circuit  is  closed  the  induced  currents  diminish 
the  negative  work  done  to  bring  the  approaching  poles 
together,  by  a quantity  which  we  will  call  D,  and  it  becomes 
— (W  — D).  When  the  poles  are  receding,  the  induced 
currents  increase  the  work  required  to  separate  them  by 
the  same  amount,  and  it  becomes  W + D. 

The  total  amount  of  work  done  in  passing  the  pole  is 
then 

W + D — (W-D)  = 2D.  . . (51) 

* The  student  who  is  unfamiliar  with  algebra  is  advised  not  to  trouble 
himself  with  this  paragraph,  as  it  is  only  a different  way  of  stating  what 
has  already  been  stated  in  words. 


J34 


E lee  trie  L ighting. 


Phases  op  an  Alternate-current  Machine.  (Plate  XVI.) 

Plate  XVI.  represents  the  phases  of  a typical  alternate- 
current  machine  during  one  complete  cycle  of  change,  i.e. 
from  the  time  a S.  pole  leaves  a given  core  to  the  time  a S. 
pole  arrives  at  it  again. 

The  three  circles  A,  B,  C,  which  in  practice  would  be 
arranged  with  others  round  a large  ring,  represent  three  coils 
of  the  machine.  The  small  central  circles  are  their  iron  cores. 
The  squares  are  the  poles  of  the  moving  magnets.  The  five 
lines  represent  the  same  three  coils  at  five  successive  phases 
of  the  cycle. 

We  will  fix  our  attention  on  the  changes  which  take 
place  in  the  centre  coil  B. 

In  position  1,  the  core  of  B has  a S.  pole  exactly  over  it, 
and  has  the  maximum  of  induced  N.  magnetism.  When  the 
magnets  begin  to  move  towards  position  2,  the  N.  polarity  of 
the  core  of  B diminishes,  as  a S.  pole  is  leaving  it  and  a 
X.  pole  approaching  it.  A current  is  therefore  generated 
in  the  direction  opposite  to  that  which  would  make  a N.  pole 
in  B,  i.e.  in  the  direction  of  the  arrows  in  line  2. 

In  position  2 the  core  has  no  polarity,  for  it  is  acted  on 
equally  by  a N.  and  by  a S.pole.  The  instant  the  magnets  have 
passed  position  2 the  core  begins  to  acquire  a S.  polarity, 
as  the  action  of  the  receding  S.  pole  is  diminishing  and 
that  of  the  approaching  N.  pole  increasing. 

The  increasing  S.  polarity  induces  a current  in  B in  the 
same  direction  as  would  make  a S.  pole  in  B,  i.e.  in  the 
same  direction  as  before.  Thus  the  current  from  position  1 
to  position  3 is  in  the  same  direction. 

In  position  3 the  S.  magnetism  has  attained  its  maximum, 
and  now  begins  to  diminish,  hence  a current  begins  to  flow 
in  the  opposite  direction,  i.e.  the  direction  shown  by  the 
arrows  in  position  4,  and  it  continues  to  flow  in  that 
direction  till  position  5 is  reached,  when  it  is  again  re- 
versed. 

Thus  we  see  that  the  direction  of  the  current  reverses  at 
positions  1,  3,  5,  and  therefore,  by  the  law  of  continuity,  it 
must  have  zero  value  in  those  positions. 


Plate  XYI. — phases  of  an  alternate  current  machine. 


Alternate  Current  Machines. 


135 


It  attains  its  maximum  value  in  what  we  will  call  the  + 
direction  at  some  position  between  positions  1 and  3,  and 
its  maximum  value  in  the  — direction,  in  some  position 
between  3 and  5. 

These  maxima  will  be,  approximately,  in  positions  2 
and  4,  but  will  not  in  general  be  accurately  in  those 
positions. 

The  positions  of  the  maxima  are  displaced  by  the  fact 
that  the  phase  of  an  induced  soft-iron  magnet'  lags  a little 
behind  the  phase  of  the  inducing  magnet,  and  owing  to 
iron  requiring  a certain  small  time  to  acquire  its  indmced 
state.  The  phase  of  an  increasing  induced  magnet  will  lag 
a little  more  than  that  of  a decreasing  one,  as  iron  gains 
magnetism  less  quickly  than  it  loses  it.  The  displace- 
ments ‘depend  on  the  size  and  hardness  of  the  iron  cores 
and  on  the  speed  of  the  moving  magnets. 

The  minima  positions  1,  3,  5 will  also  be  displaced,  but 
probably  not  so  much  as  the  maxima. 

The  maxima  are  also  displaced  by  self-induction,  as  ex- 
plained on  page  122. 

If  we  consider  the  direct  actions  of  the  magnet-poles  on 
the  wire,  we  see  that  in  the  maxima  positions  2 and  4 the 
poles  are  acting  on  both  sides  of  the  coil. 

Reaction  on  the  Magnets. 

We  now  have  to  consider  whether  the  induced  currents 
have  any  reaction  on  the  magnets,  and  if  so,  whether  it 
tends  to  strengthen  or  bo  weaken  them. 

We  know  that  any  iron  placed  near  a magnet-pole  in- 
creases its  strength.  The  poles  therefore  have  their  maximum 
strengths  in  positions  1,  3,  5,  and  their  minimum  strengths  in 
positions  2,  4.  When  no  induced  currents  are  flowing,  the 
strength  of  the  poles  decreases  in  moving  from  1 to  2 and 
from  3 to  4,  and  increases  again  in  moving  from  2 to  3 and 
from  4 to  5. 

Let  us  now  close  the  circuits  and  consider  the  actions  of 
the  induced  currents  on  the  magnets. 

It  will  be  simpler  if  we  suppose  the  magnets  to  be 


136 


Electric  Lighting. 

electro-magnets,  and  consider  the  action  of  the  induced 
currents  on  the  magnetizing  currents.  The  small  arrows 
surrounding  the  square  magnets  show  the  directions  of  the 
magnetizing  currents  producing  the  polarities  marked  on 
them. 

We  remember  that  an  increasing  current  induces  another 
in  the  opposite  direction  to  itself,  and  a decreasing  current 
one  in  the  same  direction. 

We  assume  the  electro-magnets  to  be  all  connected  to- 
gether and  magnetized  by  the  same  current,  so  that  any 
change  caused  by  an  action  on  the  current  in  any  one  of 
them  is  equally  divided  between  all,  so  that  they  remain 
equal  to  each  other. 

Now  from  position  1 to  position  2 there  is  an  increasing 
current  in  the  coil  B,  and  hence  it  induces  currents  opposite 
to  itself  in  the  coils  of  the  electro-magnets.  Hence  we  see, 
by  referring  to  Plate  XVI.,  line  2,  that  the  reaction  diminishes 
the  magnetism  of  the  N.  pole,  and  increases  that  of  the  S. 
pole. 

But  through  the  wThole  of  the  motion  from  position  1 to 
position  2 the  S.  pole  is  nearer  to  the  core  than  the  N.  pole; 
hence  the  increase  of  the  magnetizing  current  caused  by 
the  reaction  of  the  induced  current  on  the  S.  electro-magnet 
pole  is  a little  greater  than  the  decrease  caused  by  the  re- 
action on  the  N.  electro-magnet  pole,  and  hence  the  total 
effect  of  the  induced  current  between  positions  1 and 
2 is  to  cause  a slight  increase  of  the  power  of  the 
magnets. 

From  position  2 to  position  3 there  is  a decreasing  cur- 
rent in  coil  B,  and  hence  it  induces  a current  in  neighbour- 
ing coils  in  the  same  direction  as  itself.  Its  tendency  is 
thus  to  decrease  the  S.  pole  and  to  increase  the  N.  pole. 
But  during  this  portion  of  the  motion  the  S.  pole  is  further 
away  from  the  coil  than  the  N.  pole,  and  hence  the  decrease 
of  the  S.  pole  is  less  than  the  increase  of  the  N.  pole,  and 
there  is  in  this  phase  also  a small  total  increase  of  mag- 
netism caused  by  the  reaction  of  the  coil. 

In  the  same  way  an  increase  occurs  in  phases  3 to  4, 
and  4 to  5. 


Direct  Ciirrent  Machines. 


1 37 


Effect  of  Self-Induction. 

Whenever,  as  in  dynamo  machines,  the  currents  in  coils 
are  rapidly  reversed,  the  effects  of  self-induction  become 
important. 

Speaking  generally,  the  effect  of  self-induction  is  that, 
with  a given  speed,  magnet,  and  armature  coil,  the  current 
produced  is  less  than  it  would  otherwise  be. 

This  diminution,  however,  does  not,  to  the  best  of  my 
belief,  waste  energy  or  diminish  the  efficiency  of  the  machine  ; 
it  only  diminishes  its  output. 

With  the  diminution  of  output  comes  a corresponding 
diminution  of  H.P.,  so  that  if  by  running  the  machine 
faster  we  bring  the  output  up  to  what  it  would  have  been 
if  there  had  been  no  self-induction,  we  only  increase  the 
H.P.  in  the  same  proportion,  so  that  the  ratio  of  output 
to  H.P.,  i.e.  the  efficiency  of  the  machine,  remains 
unaltered. 

Thus  self-induction  increases  the  size  of  machine  required 
to  feed  a certain  number  of  lamps,  but  it  does  not  perceptibly 
increase  the  H.P.  required  to  drive  the  machine  with  that 
number  of  lamps  on  it. 

The  effect  of  self-induction  increases  as  the  current 
increases,  and  therefore  short  circuiting  a coil  of  an  alter- 
nating machine  does  not  indefinitely  increase  the  current  in 
that  coil , and  seldom  increases  it  enough  to  injure  the 
insulation. 

The  effect  of  self-induction  in  diminishing  output  can  be 
utilized  in  regulating  machines ; for  if  a number  of  coils  be 
connected  in  quantity  to  a number  of  lamps  in  quantity, 
then  cutting  out  one  or  more  coils  reduces  the  number  of 
coils  through  which  the  current  can  flow,  and  thus  increases 
the  self-induction  and  diminishes  the  E.M.F.  at  the 
lamps. 

General  Principle  of  Direct  Current  Machines. 

The  direct  current  type  of  machine  may  be  divided  into 
two  sub -types,  namely,  the  “ Gramme  33  machine  and  the 
“ Siemens 33  machine. 


138  Electric  Lighting. 

The  Gkamme  Sub-Type. 

Fig.  67  is  a diagram  of  a machine  of  the  first  sub-type. 
It  consists,  as  we  have  already  stated,  of  a ring  of  soft  iron 
round  which  wire  is  wound  as  a continuous  spiral,  forming  a 
closed  circuit. 


It  revolves  between  two  poles  of  opposite  names,  the 
lines  of  force  from  which  terminate  in  the  ring,  as  shown  in 
fig.  68,  which  represents  a section  made  through  fig.  67  by  a 
plane  in  the  line  N S of  fig.  67,  and  at  right  angles  "to  the 
plane  of  the  paper. 


//  / / / I 
/'//  I 
' / / / 
X / / / 


Fig.  68. 

As  the  ring  revolves  these  lines  of  force  are  cut  by  the 
moving  wires,  and  electro-motive  forces  are  generated  in 
the  two  halves  of  the  ring  in  opposite  directions , so  that  they 


Theory  of  the  Gramme  Machine.  139 

meet  and  oppose  one  another  at  the  neutral  points  N P, 
as  in  fig.  69. 


//  P 


As  long  as  no  further  connections  are  made,  no  current  is 
generated,  and  no  H.P.  expended.  If,  however,  the  points 
N P are  connected  through  an  external  circuit,  such  as  a 


number  of  lamps,  the  two  halves  of  the  ring  will  act  like  two 
batteries  in  parallel  circuit,  and  a current  will  flow,  as  in 
fig.  70. 


140 


Electric  Lighting. 

We  see  that,  owing  to  the  ring  being  in  motion  and  the 
neutral  point  necessarily  at  rest,  a permanent  connection 
between  the  line  and  the  wire  in  the  ring  cannot  be  made, 
but  a special  device  has  to  be  employed. 

The  Collector. 

The  collector  is  made  of  a barrel  of  wood  or  other  in- 
sulating material,  shown  in  the  centre  of  fig.  71,  on  which  are 
a number  of  insulated  metal  strips.  Each  of  these  strips  is 
connected  by  a wire  to  the  part  of  the  spiral  wire  opposite  to 
it.  Two  metal  brushes  press  or  rub  onthe  strips  at  the 
points  * P N,  where  the  opposite  electro-motive  forces  diverge 
and  join  again.  These  brushes  convey  the  current  to  the 
external  circuit. 


The  Siemens  Sub-Type. 

In  this  type  of  machine  the  wire  is  wound  longitudinally 
round  an  iron  barrel.  It  differs  from  the  Gramme  ring  by  the 
omission  of  those  parts  of  the  wire  which  pass  inside  the 
ring.  Fig.  72  shows  the  Gramme  type  in  section,  fig.  74 
the  Siemens  type,  and  fig.  73  an  imaginary  intermediate 
type. 

* These  points  are  not  usually  the  symmetrical  points,  as  shown  in 
fig.  71,  but  are  displaced  by  the  time  required  for  the  iron  of  the  ring  to 
change  its  magnetism,  as  will  be  explained  later. 


Theory  of  the  Siemens  Machine . 14 1 


The  collector  in  this  type  of  machine  is  similar  to  that  in 
the  Gramme  type. 


Fig.  72.  Fig.  73. 


Production  of  Magnetism  in  the  Field  Magnets. 

The  direct  type  of  machine  can  “ excite  ” its  own  magnets, 
as  a portion  of  the  current  generated  is  sent  round  their  coils. 
The  alternating  type  has  to  have  its  magnets  excited  by  a 
small  auxiliary  direct-current  machine.  There  are  three 
forms  in  which  the  direct-current  machines  are  constructed 
to  excite  their  own  magnets,  and  they  are  called  respectively, 
u Series-wound,”  “ Shunt-wound,”  and  “ Compound.” 

In  the  series-wound  dynamos  the  magnets  are  wound  with 
a comparatively  short  length  of  wire  thick  enough  to  carry 
the  whole  current  generated,  and  are  connected  in  series  with 
the  armature  and  brushes.  The  first  time  such  a machine 
is  worked  it  must  be  excited  separately  by  a battery  or  by 
another  machine. 

When  once  the  magnets  have  been  excited,  a feeble 


142 


Electric  Lighting . 

residual  magnetism  will  remain  in  them.  On  the  machine 
being  worked  this  feeble  magnetism  developes  a feeble 
current  in  the  armature  ring.  This  feeble  current  passing 
through  the  magnets  strengthens  them,  and  they  in  turn 
strengthen  the  current  in  the  armature.  These  alternate 
reactions  go  on  until  the  maximum  current  that  the  machine 
can  give  is  being  generated.* 

This  maximum  is  limited  first  by  the  external  resistance, 
or,  if  the  machine  be  short-circuited,  it  is  limited  by  the 
magnets  approaching  their  saturation-point,  and  by  the 
internal  resistance  of  the  armature. 

The  practical  current  that  can  be  taken  out  of  such  a 
machine  is  limited  by  the  capacity  of  the  wire  to  carry  it 
without  undue  heating. 

Short-circuiting  a series- wound  dynamo  will  do  either  one 
of  three  things  : burn  through  the  insulator,  or  by  the  extra 
H.P.  absorbed,  throw  off  the  belt  or  pull  up  the  steam-engine. 

In  shunt-wound  dynamos  the  magnets  are  wound  with  a 
large  quantity  of  thin  wire,  which  is  connected  to  the  armature 
brushes  in  quantity  with  the  lamps  or  other  external  circuit. 
The  currents  in  the  magnets  and  lamps  then  divide  ac- 
cording to  the  ordinary  rules  of  divided  circuits.  The  same 
alternate  re-inforcement  of  the  current  and  magnets  goes 
on  as  in  the  series  machines.  Short-circuiting  a shunt- 
wound  dynamo  simply  stops  the  current,  as  it  removes  all 
the  current  from  the  magnets. 

In  compound  machines  the  magnets  are  wound  partly 
shunt  and  partly  series.  These  machines  will  be  discussed  in 
the  chapter  on  the  regulation  of  dynamos. 

* The  time  required  for  the  current  to  reach  its  maximum  varies  from 
one  or  two  seconds  with  small  machines  to  about  three  minutes  in  the 
huge  Edison  machine. 


43 


CHAPTER  XT. 

ON  DESIGNING  DYNAMOS  AND  ON  THEIR  MECHANICAL 
CONSTRUCTION. 


Application  of  Mathematical  Analysis  to  Machine 
Construction. 

Engineers  are  often  disappointed  to  find  the  small  amount 
of  help  which  mathematicians  are  able  to  give  them  in 
practical  work.  For  instance,  if  we  give  to  a mathematician 
scale-drawings  of  two  forms  of  coils,  he  is  seldom  able  to 
tell  us  what  will  be  their  relative  co-efficients  of  self- 
induction. 

The  failure  of  mathematical  analysis  to  help  us  in  the 
more  complex  problems  of  our  profession  has  too  often 
caused  engineers  to  neglect  not  only  mathematical  analysis, 
but  even  arithmetical  calculation,  and  consequently  working 
empirically,  to  make  numerous  costly  and  even  dangerous 
mistakes.  Without  mathematical  reasoning  (although  it 
need  not  necessarily  be  expressed  in  symbolical  language) 
no  real  progress  can  be  made.  The  best  work  will,  however, 
be  done  by  men  who,  possessing  mathematical  knowledge, 
will  yet  not  blindly  trust  to  their  symbols,  but  will  insist  on 
knowing  the  physical  and  material  meaning,  not  only  of 
the  two  ends  of  their  analysis,  but  of  every  intermediate 
step. 

Mathematical  reasoning  is  an  invaluable  aid  to  the 
engineer,  but  whenever  he  trusts  to  mathematical  analysis 


144 


Electric  Lighting. 

instead  of  to  engineering  skill,  whenever  he  prefers  the 
manipulation  of  symbols  to  the  manipulation  of  machinery, 
disastrous  failure  will  be  the  result. 

The  reason  of  the  untrustworthiness  of  mathematical 
results  is  due  not  so  much  to  any  defect  in  the  mathematics 
as  to  the  impatience  with  which  the  purely  mathematical 
temperament  regards  the  experimental  limitations  which 
are  necessarily  present  in  practical  work,  but  which  the 
mathematician  is  not  accustomed  to  in  his  own  studies,  and 
to  the  complexity  of  the  conditions  of  the  problems  pre- 
sented to  him. 

The  mathematician  working  on  paper  is  at  liberty  to 
assume  conditions  which  cannot  be  satisfied  in  practice ; 
for  instance,  that  all  parts  of  a coil  of  wire  are  equidistant 
from  the  magnet,  and  that  parts  of  the  machine  not  intended 
to  act  on  each  other  are  at  an  infinite  distance  apart. 

Further,  the  mathematician  generally  considers  his 
result  complete  if  he  produces  a formula  connecting  the 
quantity  whose  value  is  required  with  four  or  five  “con- 
stants,” whose  value  he  assumes  to  be  known  when  he  has 
indicated  them  by  the  earlier  letters  of  the  alphabet.  As  a 
rule,  the  measurement  of  these  constants  is  a matter  of 
greater  experimental  difficulty  and  expense  than  the  direct 
determination  of  the  quantity  required. 

Lastly,  mathematicians  are  seldom  engineers,  and  it  con- 
sequently sometimes  happens  that  a machine  which  is 
excellent  electrically,  is  so  designed  that  it  could  neither  be 
constructed,  put  together,  nor  taken  to  pieces  * Perhaps  it 
has  all  its  bolts  in  positions  which  cannot  be  reached  by  the 
spanner,  or  has  its  revolving  part  supported  only  at  one  end, 
so  that  it  must  shake  and  rattle  directly  it  is  set  in  motion. 

Therefore,  as  mathematical  knowledge  is  essential,  and 
as  mathematicians  cannot  be  trusted  to  design  machines, 
the  only  way  in  which  real  progress  can  be  insured,  is  for 
practical  engineers  to  acquire  for  themselves  such  mathe- 
matical knowledge  as  they  require.  Then  their  mathematics 

* An  interesting  description  of  such  a machine,  the  invention  of  an 
eminent  mathematician,  has  been  recently  published  in  the  Patent 
Journal. 


On  designing  Dynamo -Machines.  145 

will  again  and  again  assist  them  in  their  designs,  and  their 
practical  experience  will  tell  them  when  symbolic  reasoning 
has  led  them  to  an  absurd  result. 

In  designing  a dynamo,  a first  sketch  may  be  made 
showing  the  magnets  and  armature  coils  in  their  proposed 
relative  positions.  The  proportioning  of  the  relative  sizes 
of  magnets  and  armature  coils  in  any  new  type  of  machine 
is  more  an  art  than  a science,  i.e.  it  depends  more  on  the 
individual  skill  of  the  designer  than  on  rules  which  can  be 
printed. 

It  is  a good  plan  in  designing  new  types  to  draw 
everything  full  size  “ freehand  ” on  a large  board,  and 
then,  when  satisfied  with  the  appearance,  to  measure  the 
sketch,  varying  it  as  we  go  on,  to  correct  disproportions, 
and  to  bring  the  dimensions  to  even  measurement.  Scale- 
drawings  can  then  be  made  from  the  sketch,  and  will  be 
ready  for  criticisms  based  on  measurements  of  the  quantities 
of  iron  and  copper  in  the  magnets  and  armature  as  drawn; 
for  it  is  much  easier  to  calculate  what  a machine  of  certain 
proportions  will  do,  and  to  alter  those  proportions  to  make 
it  do  something  else,  than  to  design  a machine  directly  for 
a particular  work. 

When  once  a machine  of  any  particular  type  has  been 
successfully  tried,  it  is  easy  to  design  another  of  the  same 
type,  but  of  different  size.  If  we  are  designing  a machine 
larger  than  our  model,  it  is  safe  practice  to  consider 
that  the  strength  of  magnetic  field,  the  proportions  of  the 
magnets  being  unaltered,  is  proportional  to  their  weight,  and 
that  the  proportion  of  armature  to  magnets  should  remain 
unaltered.  If  we  are  making  a machine  smaller  than  our 
model,  the  magnets  must  be  somewhat  larger  than  this  rule 
would  give.  If  the  electro-motive  force  of  the  big  machine 
is  to  be  the  same  as  that  of  the  small  one,  the  wire  in  the 
armature  must  be  proportionately  shorter  and  thicker. 

The  simplest  way  of  working  is  to  calculate  what  would 
be  the  E.M.F.  if  the  wire  had  been  the  same  as  on  the 
model,  and  then  to  calculate  the  proper  gauge  of  wire  by  the 
following  rules  : — 

(1 ) With  an  armature  coil  carrying  a given  volume  of  wire , 

L 


146 


Electric  Lighting. 

then  other  things  being  equal,  the  F.M.F.  is  proportional  to 
the  length  of  the  wire. 

(2)  The  volume  of  wire  on  an  armature  coil  is  proportional 
to  the  length  of  the  wire  multiplied  by  its  section. 

Whence — 

With  a given  armature  coil  the  F.M.F.  will  be  inversely 
proportional  to  the  section  of  the  wire. 

Example. 

The  wire  on  an  armature  coil  is  *105  inch  diameter,  and 
the  E.M.F.  is  170  volts.  What  must  be  the  diameter  to 
give  an  E.M.F.  of  100  volts  ? From  the  tables  at  the  end 
of  this  book  we  see  that  the  section  of  wire  T05  inch 
diameter  is  *00882  sq.  inch. 

The  section  of  the  desired  wire  must  be  then — 

~ X -00882  = -0149, 

and  a further  reference  to  the  tables  shows  that  this  is  a 
wire  whose  diameter  is  *138  inch. 

In  designing  a machine  larger  than  a model,  but  of 
similar  proportions,  the  output  is  a function  of  the  linear 
dimensions,  say  of  the  diameter  of  the  revolving  wheel,  and 
mathematical  analysis  shows  that  the  output  should  in- 
crease in  a ratio  between  the  4th  and  5th  power  of  the 
ratio  of  diameters,  say  the  4Jth  power. 

This  means  that  if  a machine  of  a certain  diameter  could 
feed  a certain  number  of  lamps,  one  of  twice  the  diameter 
could  feed  24^  or  22*6  times  that  number. 

In  practice,  however,  the  result  obtained  does  not  much 
exceed  the  third  power  or  cube  of  the  diameter,  which 
means  that  if  a machine  of  a certain  diameter  feeds  a 
certain  number  of  lamps,  one  of  twice  the  diameter  will  feed 
23  or  8 times  that  number.  In  other  words,  the  practical 
rule  is  that  the  output  will  be  proportional  to  the  weight  of 
the  machine. 

The  use  of  the  3rd  power  in  calculations  is  very  safe 
practice  and  allows  ample  margin,  and  the  engineer  who 
uses  it  as  the  basis  of  his  work  is  not  likely  to  be  dis- 
appointed, but  will  always  find  that  his  machines  will  do 
rather  more  than  he  has  promised  they  shall  do. 


147 


On  designing  Dynamo- Machines. 

We  may  here  note  that  an  approximate  rule  for  the 
quantity  of  wire  a bobbin  will  hold  is  that  for  moderately 
thick  wire,  double  cotton  covered,  each  cubic  inch  of  space 
will  hold  ^lb.  of  wire. 

Example. 

How  much  No.  7 wire  would  a circular  bobbin  hold, 
whose  external  diameter  is  8 inches,  internal  (i.e.  outside 
the  tube),  oj  inches,  length  between  flanges  5 inches  ? 

Area  of  circle  5 in.  in  diam.  is  19‘6  sq.  in.  (See  tables  at 
31  „ „ 96  ,,  ) end  of  book. 

Difference  10*0  „ = area  of  wire  space. 

Whence  volume  of  wire  space  = 5 x 10  = 50  cubic  inches, 

50 

and  the  weight  of  wire  will  be  = 8^  lbs. 

This  rule  is,  of  course,  a rough  one,  but  it  is  near  enough 
to  be  very  useful  in  practice,  and  is  quite  near  enough  to 
purchase  wire  by. 

The  next  stage  of  the  design  should  be  the  making  of  a 
specimen  magnet  of  the  proposed  pattern  and  size,  and  the 
examination  of  its  field  with  the  Field-measurer  already 
described.* 

Varying  currents  should  be  used  up  to  the  full  H.P. 
which  it  is  proposed  to  expend  in  the  magnet,  and  the  field 
should  be  measured  for  each  current. 

If  the  magnet  approaches  saturation  before  the  current 
has  received  its  maximum  value,  it  means  that  there  is  not 
enough  iron  in  the  magnet  in  proportion  to  the  copper  and 
to  the  H.P.,  and  the  core  must  be  made  larger. 

If  the  machine  is  one  of  a type  which  has  already  been 
tried,  then  a magnet  of  the  model  machine  may  be  placed  in 
series  with  that  of  the  proposed  machine,  and  the  same 
current  sent  through  the  two,  and  their  fields  can  be 
compared. 

For  this  experiment  the  bobbins  of  the  two  magnets  must 
be  wound  with  wire  of  the  same  gauge,  and  then  the  H.P.s 
expended  per  cubic  inch  will  be  the  same  in  both.  When 


* See  page  117,  Plate  XV. 
L 2 


148 


Elective  Lighting . 

tlie  machine  is  constructed,  any  other  gauge  of  wire  may  be 
used  that  suits  the  E.M.F.  of  the  exciting  current.  This 
change  will  not  alter  the  strength  of  field  produced  per 
H.P.  expended  in  the  magnetizing  coil. 

Armature  Coils. 

Armature  coils  (i.e.  the  coils  in  which  the  current  is  to  be 
induced)  are  made  both  with  or  without  iron  cores,  and 
there  has  been  much  controversy  as  to  which  gives  the 
better  result. 

The  use  of  iron  increases  on  the  one  hand  the  useful 
induction,  and  on  the  other  it  increases  the  useless  self- 
induction,  and  wastes  a certain  quantity  of  heat  in  the 
reversals  of  its  magnetism,  and  in  the  currents  induced  in 
itself.  The  two  last  causes  of  waste  can  be  greatly  reduced 
by  proper  annealing  and  by  suitable  slits  in  the  metals. 

My  own  experience  is  strongly  in  favour  of  the  use  of 
iron  cores,  for  both  electrical  and  mechanical  reasons. 

I approve  of  them  electrically  because  I believe  that 
the  increase  of  useful  effect  is  very  much  greater  than  the 
increase  of  waste  effect,  and  because  of  the  much  greater 
length  of  armature  coil  (measured  along  the  axis)  which 
can  be  brought  into  the  magnetic  field  when  the  lines  of 
that  field  are  concentrated  by  an  iron  core,  than  when  they 
are  diffused.  My  own  and  the  De  Meritens  alternating 
machines,  and  nearly  all  direct-current  machines,  are  made 
with  iron  cores,  and  the  alternating  machines  of  Siemens 
and  Ferranti  are  made  without  them. 

The  mechanical  advantages  of  iron  cores  are  also  very 
great,  as  they  allow  machines  to  be  strongly  constructed, 
which  is  not  possible  when  the  cores  or  other  supports  of 
the  coils  are  made  of  wood. 

As  to  the  proportions  of  armatures,  no  mathematical  rule 
can  be  laid  down,  but  a skilled  engineer  can  generally 
sketch  out  an  armature  to  work  under  any  given  circum- 
stances, which  will,  when  tried,  be  found  to  be  successful, 
and  such  that  any  deviation  from  it  will  be  less  successful. 

In  making  a machine  of  a known  type,  the  length  of 
wire  required  to  give  the  same  E.M.F.  as  the  model  is  by 
theory  inversely  proportional  to  the  product  of  the  number 


On  designing  Dynamo- Machines.  149 

of  reversals  per  minute  by  the  strength  of  the  magnetic 
field  (so  that,  for  instance,  if  the  magnetic  field  were  twice 
as  strong,  and  the  number  of  reversals  the  same  as  in  the 
model,  the  wire  in  the  armature  should  be  half  the  length 
that  it  has  in  the  model). 

In  practice  this  rule  is  not  quite  accurate,  but  has  to  be 
modified  by  practical  considerations.  The  section  of  the 
wire  is  that  which,  with  the  calculated  length,  will  just  fill 
the  armature  bobbin. 

This  rule  (as  modified)  will  give  a result  near  enough  to 
the  correct  one  to  enable  us  to  wind  one  bobbin  for  ex- 
periment. Having  tried  it  in  the  machine  and  measured 
the  E.M.F.  produced,  we  can  calculate  the  diameter  of  the 
wire,  which  will  give  us  the  exact  E.M.F.  wanted,  by  the 
rule  given  on  page  146. 

Having  drawn  the  magnets  and  coils  of  right  proportions 
and  in  their  right  positions,  we  have  to  think  how  they  are 
to  be  supported  in  those  positions,  and  how  one  or  other  of 
them  can  be  moved  rapidly  past  the  other,  and  here  we  meet 
our  first  difficulty. 

If  metal  passes  through  a magnetic  field,  electro-motive 
forces  are  induced  in  it.  If  the  metal  forms  a closed  circuit, 
these  electro- motive  forces  will  produce  currents  which  heat 
the  metal  and  consume  horse-power. 

If  the  metal  forms  a circuit  of  very  low  resistance,  the 
heating  and  horse-power  will  both  be  very  great.  If,  for 
instance,  a metal  disc  be  turned  between  the  poles  of  a 
powerful  magnet,  it  will  take  an  enormous  horse-power  to 
turn  it  rapidly,  and  the  heat  will  very  likely  be  sufficient  to 
melt  the  disc. 

In  carrying  coils  of  wire  through  a magnetic  field,  rigidity 
and  strength  are  required  in  the  supports  connecting  the 
moving  coils  to  the  driving  shaft ; rigidity,  because  it  is 
necessary  that  the  rapidly  moving  coils  should  pass  very 
close  to  the  magnet  faces,  and  strength  to  resist  the  horse- 
power pull  and  the  centrifugal  force. 

The  problem  before  us  then  is  to  support  the  moving 
coils  or  magnets  in  such  a way  that  there  shall  be  ample 
strength,  and  yet  that  currents  should  not  be  induced  in  the 
supports  of  the  armature  coils. 


i5o 


Elective  Lighting. 


As  far  as  these  conditions  are  concerned,  there  is  a great 
advantage  in  making  the  magnets  move  and  keeping  the 
coils  fixed,  as  metal  may  he  freely  used  to  carry  the  moving 
magnets,  while  the  armature  coils,  being  only  strained  by 
the  horse-power  pull,  and  not  by  centrifugal  force,  need  not 
be  nearly  so  strongly  supported  as  if  subjected  to  the 
latter,  and  the  supports,  if  of  metal,  can  be  slit  to  check  the 
circulation  of  currents.  This  plan  also  enables  the  supports 
and  cores  to  be  made  hollow  and  to  have  water  run  through 
them,  thus  increasing  the  output  that  the  machine  can  be 
made  to  give  without  undue  heating. 

It  must  be  remembered  that  the  horse-power  pull  and  the 
centrifugal  force  do  not  add  together,  as  they  act  at  right 
angles  to  each  other,  the  former  acting  along  a tangent  to 
the  revolving-wheel,  and  the  latter  along  its  radius. 


The  centrifugal  force  F in  pounds  exercised  by  any  body 
attached  to  the  rim  of  a revolving- wheel,  is  given  by  the 
formula 

F = -00034  WEN2 (52) 

where  W is  the  weight  in  pounds,*  R the  radius  in  feet,  and 
N the  number  of  revolutions  per  minute. 

Example. 

What  is  the  centrifugal  force  exercised  by  a magnet 
weighing  180  lbs.,  attached  to  the  rim  of  a wheel  2 feet  in 
diameter,  revolving  at  the  rate  of  750  revolutions  per 
minute  ? 

We  have  from  (52) — 

F = -00034  x 180  x 1 X 7502  = 34,400  lbs.  = 15*3  tons. 

The  total  strain  tending  to  pull  a wheel  into  two  halves  is 
the  total  centrifugal  force,  taking  into  account  all  the  weight 
of  the  rim,  and  of  any  weights  attached  to  it,  divided  by  7 r 
the  ratio  of  the  circumference  of  a circle  to  its  diameter  ; f or 
we  may  write  it — 


* If  W be  taken  as  the  weight  in  tons,  F will  be  the  force  in  tons, 
f 7T  = 3-1416. 


Calculation  of  Centrifugal  Force. 


(53) 


ENGINEERS  DEPARTMENT  LIBRARY 
WESTERN  ELECTRIC  COMPANY 

Centrifugdl  Force  and  Horse-power  Pull.  1 5 1 


In  calculating  the  strength  of  the  wheel,,  we  may  allow 
double  the  section  of  the  rim  at  any  point,  as  it  is  held  at 
two  points,  one  at  each  end  of  a diameter. 

Example. 

What  is  the  force  tending  to  split  a fly-wheel  into  two 
halves  when  the  diameter  is  6 feet,  weight  of  rim  one  ton, 
and  speed  100  revolutions  per  minute  ? 

We  have 


n -00034  x 3 x 2240  x 1002 
/=  Mila = 7300  lbs. 


31  tons. 


A rim  of  this  weight  and  diameter  would  have  a cross 
section  of  about  38  square  inches,  but  the  cross  section 
available  for  resisting  the  strain  would  be  double  this,  or 
76  square  inches. 

Horse-Power  Pull. 

When  we  know’  the  horse-power,  the  speed,  and  the 
diameter  of  the  wheel,  we  can  calculate  the  tangential 
strain  as  follows 

Let  us  suppose  that,  instead  of  carrying  coils  round  and 
generating  electricity,  a wheel,  of  the  diameter  which  the 
wheel  of  the  dynamo  has  at  the  centre  of  the  coils,  is  ex- 
pending the  same  horse-power,  by  winding  a string  round 
its  rim  and  raising  a weight,  then  this  weight  will  be  equal 
to  the  tangential  strain. 

Let  Y be  the  velocity  of  the  rim  in  feet  per  minute,  then 
Y = ttDN (54) 

where  D is  the  diameter  in  feet,  and  N the  number  of 
revolutions  per  minute. 

We  know  that  a horse-power  equals  33,000  foot-pounds 
per  minute,  and  the  number  of  pounds  which  one  horse- 
power will  raise  in  a minute  is  therefore  33,000  divided  by 
the  height  in  feet  to  which  the  weight  is  raised. 

The  weight  W in  pounds  which  a horse-power  can  raise 
at  a speed  Y (where  Y would  be  the  rate  at  which  the 
string  would  be  wound  round  the  wheel)  is  then 
_ 33^000 
Y 


. (55) 


152  Electric  Lighting. 


or  substituting  the  value  of  V from  (54),  we  get 


W 


33,000 

ttDN 


. (56) 


and  the  weight;  which  any  number  H.P.  of  horse-power 
could  raise  under  the  same  circumstances  would  be 


w _ H.P.  x 33,000 
VV  ~ V = - 


H.P.  x 33,000, 
ttDN 


(57) 


and  this  weight  is  the  total  tangential  strain  at  the  rim  of 
the  wheel. 

Having  got  the  total  tangential  strain,  then  dividing  the 
result  by  the  number  of  coils  in  the  rim,  gives  us  the 
strain  on  each  coil. 

Example. 

In  a dynamo  having  128  armature  coils,  a wheel  8 feet  in 
diameter  is  revolving  180  times  a minute,  and  absorbing 
500  H.P.,  what  is  the  tangential  strain  in  each  armature 
coil  ? 

The  total  tangential  strain  is  from  (57)  — 


_ 500  x 33,000 
W 314  x 8 x 180 


3640  lbs., 


and  the  strain  on  each  coil  is 


W 

128 


= 28-4  lbs. 


Factor  of  Safety. 

The  factor  of  safety  in  any  work  is  the  ratio  of  the 
calculated  breaking  strain  to  the  actual  strain  which  the 
work  is  subjected  to.  For  instance,  if  a rod  which  would 
break  with  a weight  of  15  tons,  supported  a weight  of  3 tons, 

15 

its  factor  of  safety  under  that  load  would  be  — = 5. 

In  dynamo  work  the  strains  are  so  sudden  and  the 
strengths  are  so  altered  by  unequal  heating,  that  the  factor 
of  safety  should  be  very  large ; never  less  than  15  in  the 
moving  parts,  and  10  in  the  parts  which  are  at  rest.  Indeed 
in  my  own  practice  I never  use  less  than  20  in  the  moving 
parts. 


Factor  of  Safety. — Speed. 


153 


Strength  of  Materials. 


The  strengths  of  various  materials  per  square  inch  section 
are  given  in  the  appendix  to  this  book. 


Use  of  the  Factor  of  Safety  in  Designing. 

We  wish  to  know  how  many  square  inches  of  metal  are 
required  for  safety  under  given  conditions  of  strain. 

Let  F be  the  straining1  force  (centrifugal  or  otherwise)  in 
tons,  (f>  the  factor  of  safety,  and  B the  breaking  strain  of 
the  material  in  tons  per  square  inch,  then  A the  required 
area  or  section  will  be 


(58) 


Example. 

Let  us  suppose  we  have  a wrought-iron  disc,  3 inches 
thick  and  2 feet  diameter  at  magnet  centres,  and  that  we  are 
fixing  magnets  round  its  rim  by  passing  their  cores  through 
holes  drilled  in  the  2 foot  circle.  Speed  to  be  750  revolu- 
tions per  minute.  Weight  of  each  magnet,  180  lbs.  What 
should  be  the  radius  of  metal  outside  the  holes  to  give  a 
factor  of  safety  of  20  ? 

By  the  example  given  on  page  150,  with  the  same  data, 
the  centrifugal  force  will  be  F = 15’ 3 tons,  we  have  <£  = 20, 
and  we  may  take  B at  25  tons  per  square  inch. 

Thus,  by  (58)  the  area  must  be 


A 


15-3  x 20 
25 


12-3  square  inches. 


But,  in  order  for  the  magnet  to  tear  out,  it  must  shear 
two  faces,  for  it  must  cut  out  a piece  from  the  rim  as  wide  as 
itself.  Hence  the  area  of  each  face  will  be  6*15  square  inches. 
The  thickness  being  3 inches,  this  would  give  the  necessary 
metal  outside  the  magnet-hole  at,  say  2J  inches,  measured 
along  the  radius. 

Speed. 


The  faster  the  dynamo  runs  the  greater  will  be  its  output, 
and  the  output  will  be  nearly  proportional  to  the  speed.  It 
therefore  appears  at  first  sight  that  dynamos  should  be  run 


154 


Electric  Lighting . 

as  fast  as  possible.  It  is  usually  noticed  that  when  elec- 
tricians who  are  not  skilled  engineers  commence  to  design 
dynamos  they  make  them  to  run  at  immense  speeds,  and 
consequently  fail  to  produce  useful  machines.  Most  of  the 
breakdowns  which  have  caused  the  public  to  distrust  electric 
light  have  been  due  to  dynamos  being  run  at  too  high  a speed. 
When  first  I commenced  to  design  dynamos,  I fell  into  the 
common  error  of  supposing  that  great  speeds  were  advan- 
tageous, and  in  an  article  on  “ Electric  Lighting/5  which  I 
communicated  to  the  Quarterly  Review  for  October,  1881,  I 
supported  this  view,  and  it  was  not  until  I had  had  one  of 
my  high-speed  machines  fly  to  pieces,  with  the  result  of 
wrecking  the  portion  of  the  factory  in  which  it  stood,  and 
destroying  the  result  of  nine  months5  labour,  that  I modified 
my  views. 

It  must  be  remembered  that  the  annual  cost  of  a dynamo 
is  not  its  first  cost,  but  is  the  interest  and  depreciation  on 
that  first  cost,  and  that  the  depreciation  rate  per  cent,  is 
much  greater  at  high  speeds  than  at  low  ones.  By  increasing 
the  speed  we  diminish  the  first  cost  per  lamp,  but  we  may 
so  much  increase  the  depreciation  rate  that  the  annual  cost 
is  greater  than  before,  and  in  addition  we  have  the  liability 
to  break  down  and  put  out  the  lights  suddenly. 

For  example,  if  for  1000  lights  we  have  two  dynamos 
running  at  low  speed,  and  costing  £500  each,  the  total  first 
cost  will  be  £1000,  and  we  may  take  the  interest  at  5 per 
cent,  and  depreciation  2 1,  which  will  make  the  total  annual 
cost  of  the  dynamos  themselves  7J  per  cent,  on  £1000,  or 
£75  annually. 

Now  let  us  suppose  that  we  double  our  speed ; we  shall 
be  able  to  do  with  only  one  dynamo,  and  the  total  first  cost 
will  be  only  £500.  We  may  take  the  interest  as  before  at 
5 per  cent.,  but  the  depreciation  will  not  be  less  than  15  per 
cent.,  making  the  total  annual  cost  of  the  dynamo  itself 
20  per  cent,  on  £500,  or  £100  annually. 

Further,  it  must  be  remembered  that  the  dynamos  do  not 
represent  more  than  about  20  per  cent,  of  the  whole  cost  of 
an  electric  light  plant  (i.e.  boilers,  engines,  dynamos,  mains, 
&c.),  so  that  a saving  of  half  the  cost  of  the  dynamos  only 


Speed  of  Dynamo-Machines . 155 

saves  10  per  cent,  on  the  whole  cost.  On  the  other  hand, 
the  dynamo  is  the  very  heart  and  lungs  of  the  whole  system, 
and  any  defect  in  it  will  render  the  whole  system  useless. 

For  all  these  reasons  I am  of  opinion  that  the  speed  of 
dynamo  should  be  limited  by  mechanical  considerations  to 
one  that  will  not  rattle  or  shake  it,  or  raise  the  depreciation 
above  a very  low  annual  rate. 

It  must  be  remembered  that  dynamo  machines  in  no  way 
differ  from  other  machines  absorbing  horse-power,  and  that 
when  a large  horse-power  is  being  received  by  any  machine, 
that  machine  must  be  of  a certain  size  and  strength  to  stand 
the  strain,  caused  by  it.  In  order  to  remind  himself  that 
the  driving  of  dynamos  is  subject  to  the  same  conditions  as 
the  driving  of  other  machinery,  the  engineer  would  do  well 
to  consider  what  would  happen  if  in  the  dynamo  that  he  has 
designed  the  moving  coils  were  removed,  and  a wheel 
retarded  by  a brake  substituted,  the  brake  being  so  loaded 
that  if  the  wheel  were  running  at  the  same  speed  as  the 
wheel  carrying  the  coils  would  have  done,  the  same  horse- 
power would  be  absorbed  as  would  be  absorbed  in  the 
dynamo. 

He  will  further  do  well  to  make  his  dynamo  a good  deal 
stronger  than  would  appear  to  be  necessary  even  under  these 
conditions. 


Electric  Lighting. 


J56 


CHAPTER  XII. 

SOME  TYPICAL  ALTERNATING-CURRENT  MACHINES. 


The  De  Meritens  Magneto-Machine. 

The  first  machine  which  we  shall  describe  is  the  magneto 
machine  of  M.  de  Meritens.  The  machine  consists  of  one 


Fi 75. 


The  De  Meritens  Magneto- Machine.  15  7 

or  more  rings  carrying  coils,  revolving  between  the  poles  of 
permanent  steel  magnets. 

Fig.  75  shows  a one-ring  machine  of  this  form. 

The  coils  consist  of  iron  cores  wound  with  wire  of  the 
form  shown  in  fig.  76.  They  are  mounted  on  a light  brass 


Fig.  76. 


ring,  and  are  held  to  it  by  brass  pins  which  pass  through  the 
lugs  and  through  grooves  in  the  ends  of  the  cores,  as  shown 
in  fig.  76,  so  that  the  cores  do  not  touch  each  other,  but  are 
magnetically  insulated.  When  the  machine  is  at  rest,  there 
is  a magnet-pole  of  alternately  opposite  name  over  each 
junction,  as  seen  in  figs.  75  and  77.  As  the  wheel  revolves,  the 


Fig.  7 7. 


polarities  of  the  cores  are  constantly  reversed,  and  currents 
are  therefore  induced  in  the  wires. 


158  Electric  Lighting. 

Plate  XVII.  shows  aDe  Meritens  machine  with  five  rings. 
We  note  that  the  magnets  are  here  placed  radially  instead 
of  parallel  with  the  shaft,  as  in  fig.  75. 

Fig\  78  is  a section  through  the  five-rip g machine,  showing 
the  general  arrangement. 


1'.4* >: 


Fig.  78. 


A reference  to  figs.  75  and  77  shows  us  that  by  means  of  the 
projecting  ends  of  the  cores,  the  latter  are  brought  very  close 
to  the  poles  of  the  magnets,  and  so  the  latter  can  exercise 
their  maximum  effect. 


The  Cores. 

The  cores  consist  of  a great  number  of  plates  of  very 
thin  sheet  iron,  lightly  welded  together,  so  as  fco  form  a 
solid  block  the  full  size  of  the  coil.  Iron  is  then  cut  away 
by  suitable  machinery,  so  as  to  form  the  wire  space.  The 
softer  the  iron  of  an  armature  core  is,  and  the  more  separate 
strips  it  consists  of,  the  more  easily  it  reverses  polarity. 
Each  strip  must  be  the  full  length  of  the  core. 

The  Connections. 

The  wire  is  wound  in  the  same  direction  round  all  the 
coils,  but  as  the  polarity  of  the  cores  is  alternately  in 
opposite  directions,  the  directions  of  the  currents  induced  in 


Plate  XVII. — the  de  meritens  magneto  machine. 


The  Siemens  Alternating  Machine.  159 

neighbouring  coils  are  in  opposite  directions,  and  conse- 
quently the  connections  have  to  be  made  as  in  fig.  79. 


By  means  of  a plug-board  which  revolves  with  the 
wheels,  and  which  is  seen  in  Plate  XVII.,  the  coils  can  be 
grouped  in  series,  quantity,  or  any  combination  of  the  two, 
according  as  to  whether  currents  of  high  or  low  E.M.F. 
are  required. 

The  currents  are  taken  off  by  means  of  springs  pressing 
on  metal  rings  revolving  with  the  shaft,  but  insulated  from 
it,  and  connected  to  the  different  rings  of  coils  respectively. 
The  large  machine  will  thus  feed  five  separate  circuits,  or 
any  two  or  more  of  the  circuits  can  be  connected  into  one. 

The  De  Meritens  machine  is  excellent  in  working,  and  its 
construction  is  good  both  mechanically  and  electrically ; its 
first  cost,  however,  is  very  heavy.  It  is  much  used  for 
lighthouses,  where  first  cost  is  unimportant,  and  where  its 
extreme  simplicity  and  non-liability  to  break  down  recom- 
mend it. 

The  Siemens  Alternating  Machine. 

The  Siemens  alternating  machine,  fig.  80,  consists  of  two 
fixed  iron  rings  carrying  electro-magnets.  These  magnets 
are  excited  by  a small  auxiliary  direct-current  machine. 
The  polarity  of  the  magnets  in  each  ring  is  alternately 
X.  and  S.,  and  the  polarity  of  each  is  opposite  to  that  of 
the  magnet  opposite  to  it  on  the  other  ring.  Each  magnet 
has  an  extended  flat  pole-plate,  as  shown. 

Between  the  two  rings  of  magnets  revolves  a wheel, 
partly  of  wood,  partly  of  metal,  carrying  in  its  circum- 
ference a number  of  coils  equal  to  the  number  of  magnets 


i6o 


Electric  Lighting . 

in  each  ring.  A.s  the  wheel  revolves,  currents  are  induced 
in  these  coils  in  the  manner  explained  in  page  134.  The 
currents  are  taken  off  by  springs  and  insulated  contact 
rings  in  the  same  manner  as  in  the  De  Meritens  machine. 


Fig.  80. 


The  coils  have  no  iron  cores,  and  are  wound  in  brass  bobbins, 
whose  flanges  are  perforated  with  holes  to  check  the 
circulation  of  currents  in  them. 

The  Ferranti  Machine. 

The  Ferranti  machine  externally  resembles  the  alter- 
nating Siemens,  that  is,  the  two  rings  of  fixed  magnets  are 
similar  to  those  of  the  Siemens  machine,  but  the  revolving 
armature  is  different.  Instead  of  a number  of  coils  of  wire 
fixed  to  the  rim  of  a wheel,  it  consists  of  a continuous  zigzag 
of  copper  ribbon  arranged  as  in  fig.  81,  so  that  the  alternate 
radial  portions  are  opposite  poles  of  alternate  polarity. 
Thus  the  E.M.F.s  in  alternate  portions  point  to  and  from 
the  centre  of  the  machine  alternately,  and  therefore  are  at 


The  Ferranti  Machine. 


16 1 


any  instant  all  in  the  same  direction  in  the  ribbon.  Half 
way  between  each  pole  the  current  reverses,  and  so  an 
alternating  current  is  produced  at  the  contact  rings,  where 
it  is  taken  off  in  the  same  manner  as  in  the  De  Meritens 
and  Siemens  machines. 

No  iron  is  used  in  the  armature,  and  the  latter  being  very 
thin,  the  opposed  magnet-poles  can  be  brought  very  close 
together,  and  so  the  field  of  force  is  very  intense. 


The  machine  is  driven  at  an  enormous  speed,  up  even  to 
2000  revolutions  per  minute,  and  consequently  the  quantity 
of  electricity  produced  by  a machine  of  a given  size  is  very 
large. 

The  design  of  the  machine  is,  however,  essentially  an 
electrician’s  design  as  opposed  to  an  engineer’s  design ; elec- 
trically the  machine  is  admirable,  mechanically  I venture  to 
think  that  it  is  impracticable. 

It  is  an  essential  point  of  the  electrical  design  that  no 
metal  other  than  the  copper  ribbon  should  move  through 
the  magnetic  field.  This  prevents  the  ribbon  being  sup- 
ported on  a metal  wheel,  and  thus  all  the  centrifugal  force 
and  vibration  consequent  on  the  high  speed,  and  all  the 
tangential  pull  consequent  on  the  concentration  of  a great 
deal  of  horse-power  in  a small  space,  have  to  be  borne  by  a 
zigzag  copper  ribbon,  covered  with  a more  or  less  soft  insu- 
lator, and  tied  on  to  a wooden  hub. 


162 


Electric  Lighting . 

In  discussing  dynamo  machines  our  critical  faculties  are 
apt  to  be  blunted  by  a half-acknowledged  belief  that 
electrical  forces,  being  different  to  forces  which  we  have 
been  accustomed  to,  are  also  unique  in  their  relations  with 
ordinary  mechanical  forces.  This  is  not  the  case,  and  the 
right  way  to  criticize  the  construction  of  a dynamo  is  to 
consider  what  would  happen  if  it  were  run  at  its  full  speed 
without  the  magnets  being  excited,  and  a mechanical 
retarding  force,  such  as  a brake,  applied  to  the  moving 
coils,  requiring  the  same  horse-power  to  overcome  it  as 
the  electrical  energy  which  the  machine  is  intended  to 
generate. 

I do  not  pretend  to  say  that  very  high-speed  dynamos, 
even  with  wooden  wheels,  may  not  work,  and  work  for  a 
considerable  time,  without  breaking  down,  but  I consider 
that  such  machines  cannot  be  trusted  for  large  plants  or  for 
central  station  work.  It  is  not  sufficient  to  be  able  to  say 
that  a machine  will  probably  be  safe,  but  it  is  absolutely 
essential  that  a break-down  should  be  impossible. 

To  satisfy  this  condition  I believe  that  slow  speed  is 
essential,  and  that  wrought-iron,  steel,  or  phosphor-bronze 
are  the  only  materials  which  are  admissible  in  the  construc- 
tion of  the  wheel  which  carries  the  moving  coils. 

O 


The  Gordon  Machine. 

I have  therefore  constructed  a machine  in  the  design  of 
which  I have  endeavoured  to  keep  the  above  conditions  in 
my  mind.  As  to  the  correctness  of  these  views  I can  only 
say  that  after  fifteen  months’  experience  of  the  machine,  I 
have  reason  to  be  satisfied  with  its  performance,  and  that 
my  opinion  is  daily  strengthened  that  it  is  only  by  the 
use  of  colossal  machines  that  electric  lighting  on  a serious 
scale  can  be  carried  out,  though  I would  not  for  a moment 
deny  that  numbers  of  small  machines  work  very  nicely  when 
doing  small  work. 

In  these  machines  the  magnets  revolve  and  the  coils  are 
fixed.  This  alteration  of  the  ordinary  practice  has  been 
resolved  on  for  three  reasons.  First,  as  the  magnetic  field 


' 

' 


Plate  XVIII. — the  Gordon  dynamo  machine. 


The  Gordon  Machine. 


163 


revolves  with,  the  moving  wheels  instead  of  the  latter  passing 
through  it,  the  wheel  can  be  made  of  massive  wrought-iron 
plates,  thus  giving  great  strength  and  freedom  from  vibra- 
tion. 

2nd.  The  armature  coils  being  fixed,  the  current  is  taken 
off  without  rubbing  contacts,  which  latter  are  always  a 
source  of  trouble  with  powerful  currents. 

3rd.  The  heaviest  part  of  the  machine  revolves,  and  so 
acts  as  a very  efficient  flywheel,  keeping  the  light  steady,  in 
spite  of  slight  irregularities  in  the  engine.  Two  sizes  of 
the  machine  have  as  yet  been  constructed,  the  eight-foot 
machine,  which  it  is  calculated  will,  with  sufficient  steam- 
power,  work  5000  lights  of  twenty  candles  each,  and  the 
two-foot  machine,  which  works  800  to  1000  lights  without 
any  undue  strain.  The  machines  are  made  in  the  works  of 
the  Telegraph  Construction  and  Maintenance  Company, 
at  Greenwich. 

The  Eight-Foot  Machine.  (Plate  XVIII.) 

This  machine  was  the  first  constructed ; it  consists  of  a 
wrought-iron  wheel,  carrying  the  magnets,  which  is  eight 
feet  diameter  at  the  magnet  centres  (8  ft.  9 in.  over  all). 
The  magnets,  which  are  thirty-two  in  number,  consist  each 
of  a cylindrical  core  of  soft  wrought-iron,  which  passes  right 
through  the  wrought-iron  disc,  and  projects  equally  on  both 
sides  of  it. 

Brass  bobbins,  containing  the  magnet  wire,  are  slid  on  to 
the  projecting  portions  of  the  cores,  and  are  kept  in  their 
places  by  the  pole  plates,  which  are  afterwards  attached.  The 
revolving  wheel  is  built  up  of  sheets  of  boiler-plate  rivetted 
together,  and  strengthened  by  two  cones  of  boiler-plate, 
placed  one  on  each  side.  The  cones  and  disc  are  separated 
by  cast-iron  distance-pieces,  as  shown  in  section  in  fig.  82. 

This  wheel  revolves  between  two  fixed  iron  rings  carrying 
the  armature  coils. 

Each  ring  carries  twice  as  many  armature  coils  as  there 
are  magnets  on  the  ring.  The  reason  of  this  is  the  follow- 
ing : In  an  early  model  which  was  made  of  the  machine  the 

m 2 


164 


Electric  Lighting. 

number  of  armature  coils  on  each  ring  was  made  the  same 
as  the  number  of  magnets.  It  was  then  found  that  the 
mutual  induction  of  neighbouring  coils  very  greatly  dimi- 
nished the  output,  i.e.  that  if  one  coil  was  at  work,  and 
then  the  coils  on  each  side  of  it  were  set  to  work,  that 
the  output  of  the  first  coil  was  reduced  by  nearly  fifty 
per  cent. 


The  reason  of  this  was  that  the  currents  in  the  neighbour- 
ing portions  of  adjacent  coils  were  in  the  same  direction,, 
and  hence  diminished  each  other  by  their  mutual  induction, 
in  the  same  way  that  the  currents  in  different  windings  of 
the  same  coil  diminish  each  other  by  self-induction. 


The  Gordon  Machine, 


i65 

The  plan  now  adopted  (of  making  tlie  number  of  arma- 
ture coils  double  the  number  of  the  magnets)  obviates  this 
defect,  as  at  any  instant  when  the  coils  1,  3,  5,  &c.,  number- 
ing round  the  circle,  are  producing  their  maximum  current, 
the  coils  2,  4,  6,  &c.,  are  idle.  Immediately  afterwards  the 
coils  2,  4,  6,  &c.,  will  be  at  work,  and  1,  3,  5,  &c.,  idle,  and 
thus  each  two  active  coils  are  always  separated  by  an  idle 
one  which,  partly  by  the  space  it  occupies  and  partly  by  its 
shielding  action,  so  reduces  the  mutual  action  of  the  coils  on 
either  side  of  it,  as  to  render  it  inappreciable.  Their  actions 
on  the  intermediate  coil  are  equal  and  opposite,  so  produce 
no  current  or  change  of  current  in  it. 

The  fixed  coils  are  secured  to  cast-iron  frames,  but  the 
cores  are  prolonged,  so  that  the  frames  are  set  back  into  a 
field  of  weak  magnetism.  Fig.  83  shows  some  of  the  fixed 


Fig.  83. 

coils.  Their  flanges  are  made  of  German  silver  to  check 
the  circulation  of  currents  in  them. 

The  eight-foot  machine  runs  at  140  to  180  revolutions 
per  minute,  and  so  is  connected  direct  to  the  steam-engine 


i66 


Electric  Lighting . 

without  belting.  The  total  weight  is  twenty-two  tons,  and 
the  total  weight  of  tlie  revolving  magnet  wheel  seven  tons. 
The  only  rubbing  contact  is  that  where  the  exciting  current 
produced  by  two  Burgin  machines  enters  the  revolving 
magnets.  The  method  of  regulation  will  be  given  in 
chapter  XIV. 


167 


CHAPTER  XIII. 

SOME  TYPICAL  DIRECT-CURRENT  MACHINES. 


I.  The  Gramme  Sub -Type. 

The  Crompton-Burgin  Machine. 

One  of  the  best  machines  of  the  Gramme  sub-type,  now  in 
practical  use,  is  that  invented  by  Mr.  Bur  gin,  and  improved 
upon  by  Mr.  Crompton.  Plate  XIX.  gives  two  views  of  one 
of  these  machines  of  the  size  which  works  eighty  20-candle 
incandescent  lamps,  or  four  large  arc  lamps. 

The  revolving  portion  of  the  machine  is  shown  at  the 
bottom  of  Plate  XX.  It  consists  of  four  or  more  Gramme 
rings  mounted  on  the  same  shaft,  each  consisting  of  a coil 
of  iron  wire  of  a hexagonal  shape.  The  copper  wire  is 
wound  on  the  flat  portions  of  each  hexagonal  ring,  leaving 
the  corners  bare,  and  more  layers  are  wound  at  the  centre 
than  at  the  ends  of  each  side. 

Thus  a line  drawn  round  the  ring  outside  the  copper  wire 
is  very  nearly  circular.  In  Plate  XX.  one  side  of  one  ring 
has  been  shown  without  any  wire  on  it  for  clearness.  The 
rings  are  so  fixed  on  the  axis,  that  each  gains  a little  on  the 
next  one,  so  that  lines  joining  corresponding  angles  of  the 
hexagons  would  form  spirals. 

The  collector,  seen  ou  the  left  of  the  lower  figure  in 
Plate  XX.,  and  also  in  the’  centre  of  the  lower  figure  in 
Plate  XIX.,  consists  of  a great  number  of  strips  of  copper, 
insulated  from  each  other  and  fixed  on  an  insulating  barrel. 


i68 


Electric  Lighting. 

Each  metal  strip  is  attached  to  the  corresponding  portions 
of  the  coils  of  copper  wire  on  each  of  the  four  rings.  The 
current  is  collected  by  means  of  the  brushes  seen  in  the 
lower  figure  in  Plate  XIX. 

The  magnets,  which  also  form  the  framework  of  the 
machine,  are  shown  in  the  upper  figure  of  Plate  XX.  They 
are  of  cast-iron,  and  the  upper  and  lower  portions  are  cast 
separately.  The  two  portions  having  been  bolted  together, 
the  cylindrical  part  in  the  centre,  in  which  the  armature  to 
revolve  is  turned  or  bored  true. 

The  arms  of  the  magnets  are  wound  with  wire,  as  shown 
in  Plate  XIX.,  and  so  connected  that  one  of  the  circular 
segments  is  a N.  pole,  and  the  other  is  a S.  pole. 

The  bearings  in  which  the  shaft  runs  are  carried  by  frames 
of  brass  or  other  non- magnetic  metal,  screwed  on  to  the 
ends  of  the  pole-pieces.  These  bearings  are  made  whole, 
and  not  in  two  halves  as  in  most  machinery. 

In  putting  the  machine  together,  the  magnets  are  first 
bolted  together,  then  the  brass  at  the  collector  end  is  screwed 
on.  Next,  the  revolving  armature  is  slid  in  from  the  other 
end,  and  the  collector  end  of  the  shaft  slid  into  its  bearing. 

Then  the  second  brass  is  slid  over  the  pulley  end  of  the 
shaft  and  bolted  to  the  pole-pieces,  and  last  of  all  the  pulley 
is  put  on  and  secured  by  a set-screw  pressing  on  the  flat- 
tened end  of  the  shaft. 

The  collecting  brushes  are  carried  by  a moveable  arm, 
which  can  be  turned  round  the  bearing  at  the  collector  end 
of  the  machine,  the  outside  of  the  brass  being  turned  true 
for  the  purpose.  It  can  be  clamped  at  any  angle  by  a set- 
screw, seen  just  below  the  end  of  the  shaft  in  the  lower  figure 
of  Plate  XIX. 

The  reason  of  having  this  adjustment  is  that  the  points 
where  the  E.M.F.s  of  the  two  halves  of  the  ring  meet 
(P  N,  fig.  71,  page  140)  are  not  the  symmetrical  points 
shown  in  that  figure,  but,  by  reason  of  the  appreciable  time 
taken  to  change  the  magnetism  of  the  iron  cores  of  the  re- 
volving rings,  are  displaced  forward,*  and  the  brushes  have  to 

# ‘‘Forward”  means  in  the  direction  in  which  the  armature  is 
revolving. 


Plate  XIX.— the  crompton-burgin  dynamo  machine. 


The  Crompton-Burgin  Machine.  169 

be  moved  forward  to  follow  them.  These  points,  which  are 
the  points  of  least  sparking,  have  to  be  found  experimentally, 
by  moving  the  brushes  while  the  machine  is  running.  This 
can  be  done  by  means  of  the  arm  we  have  just  described. 

The  brushes  consist  of  a number  of  strips  of  thin  sheet- 
copper  about  two  inches  wide,  which  are  held  in  a brass 
frame.  They  slowly  wear  away,  but  can  be  pushed  forward 
by  loosening  the  clamp  screw  at  the  back  of  the  holder  shown 
in  the  lower  figure  of  Plate  XIX.,  and  when  consumed  can  be 
renewed  at  small  expense. 

The  brushes  are  pressed  upon  the  collector  barrel  by 
means  of  springs,  and  can  be  lifted  off  it  when  the  machine 
is  not  in  use  by  means  of  little  levers. 

The  rods  carrying  the  brushes  are  insulated  from  the 
adjusting  arm,  and  are  connected  to  the  terminals  at  the  top 
of  the  machine  by  means  of  flexible  wires. 

These  machines  are  wound  “ series/’  ee  shunt,”  or  “ com- 
pound,”* according  to  the  work  for  which  they  are  intended, 
They  run  at  1600  revolutions  per  minute. 

The  working  of  these  machines  is  extremely  satisfactory, 
the  only  repairs  which  they  require  are  due  to  the  fact  that 
the  sparking,  which  is  incident  to  all  direct-current  machines, 
is  apt  to  wear  the  collector  barrel  into  grooves.  I find  it 
advisable,  when  these  machines  are  in  nightly  use,  to  take 
out  the  armature  and  place  it  in  the  lathe  about  every  two  or 
three  months,  and  to  take  a very  light  cut  over  the  collector 
barrel  with  a sharp -pointed  tool. 

The  Brush  Machine.  (Plate  XXI.) 

This  machine  may  be  considered  as  a development  of  the 
Gramme  sub-type,  but  differs  from  the  Gramme  machine  in 
many  important  particulars. 

The  revolving  armature  consists  of  a wrought-iron  ring, 
figs.  84  and  85,  round  which  the  wire  is  wound  in  the  hollow 
channels  seen  in  fig.  84,  so  that  it  forms  the  coils  seen  in 
fig.  85.  The  iron  between  the  coils  comes  up  close  to  the 
magnet-poles,  and  so  receives  an  intense  magnetism.  The 


* See  Chap.  XIV. 


170 


Electric  Lighting 


Fig.  85. 


Plate  XXI. — the  brush  dynamo  machine 


' 


. 


The  Brush  Machine.  1 7 1 

annular  channels  are  cut  to  prevent  the  circulation  of 
currents  in  the  iron  itself. 

The  ring*  is  mounted  on  spokes  and  on  a hub  of  German 
silver  (not  shown  in  the  figs.),  the  high  resistance  of  which 
prevent  the  induced  E.M.F.s  making  serious  currents  in  it. 

It  revolves  between  magnets  with  extended  pole-pieces, 
as  shown  in  Plate  XXI.,  the  two  opposed  poles  at  the  same 
side  of  the  ring  being  of  the  same  name. 

So  far  the  machine  is  closely  allied  to  the  Gramme,  but 
the  method  of  collecting  the  currents  is  peculiar  to  itself. 

In  the  machine  we  are  now  discussing,  the  coils  are  eight 
in  number,  and  each  is  connected  in  series  to  the  one  opposite 
to  it,  i.e.  at  the  other  end  of  a diameter  of  the  ring.  Thus 
the  eight  coils  form  four  separate  circuits,  each  circuit  having 
its  own  commutator. 

The  four  commutators  are  arranged  in  two  pairs,  the  two 
of  one  pair  commutating  alternate  coils  with  the  two  of  the 
other  pair. 


Fig.  86. 

In  the  first  machines  designed,  each  commutator  consisted 
of  a pair  of  circular  metal  segments  (A  B,  fig.  86)  to  which 
the  two  free  ends  (P  N,  fig.  86)  of  its  coils  were  respectively 
attached.  The  brushes  +Z>,  — b pressed  on  these  rings. 


172 


Electric  Lighting. 

As  the  armature  ring  revolves,  the  current  in  the  coils 
1,1  a (fig.  87)  reverses  twice  each  revolution,  - and  at  the 
moment  of  reversal  the  segment  A (fig.  86),  which  was  in 
contact  with  the  positive  brush  ( + &),  left  it,  and  the  seg- 
ment B arrived  at  it,  and  hence  the  alternating  current 
generated  in  the  coils  was  received  as  a direct  current  in  the 
brushes  +b,  —b. 

Close  to  the  segments  A B (fig.  86)  on  the  shaft  attached  to 
the  coils  1,  la  (fig.  87),  were  a similar  pair  of  segments,  which 


1 


we  will  call  A'  B',  attached  to  the  coils  3,  3a.  The  division 
line  of  these  segments  was  at  right  angles  to  the  division  line 
of  the  first  pair. 

The  brushes  + b,  — b,  are  wide  enough  to  rest  on  both 
pairs  at  once.  When  the  machine  is  working,  the  current 
in  each  pair  of  coils  commences  as  the  coils  leave  the 
neutral  point,  rapidly  reaches  its  maximum,  and  continues 
steady  till  the  coils  approach  the  neutral  point  at  the  other 
side,  when  it  again  rapidly  falls  to  zero.  Thus  the  wide 
brush  connects  the  coils  1,  la  and  3,  3a  “in  quantity,”  and 
as  the  current  in  one  is  at  a maximum  while  the  other  is 
a minimum,  the  total  current  has  a nearly  constant  value. 

Here,  however,  a difficulty  occurred.  When  one  pair, 
say  1,  la,  were  in  the  maximum  position,  there  was  hardly 
any  E.M.F.  being  induced  in  the  other  pair,  3,  3a,  and  as 
they  were  connected  in  quantity  with  the  first  pair,  they 


The  Brush  Commutator . 


173 


formed  a shunt  to  the  lamps,  and  allowed  a back  current  to 
flow  through  them  from  the  active  coils  connected  with  them. 
To  get  over  this  difficulty,  Mr.  Brush  shortened  the  seg- 


mental pieces  A B of  the  commutators,  and  introduced  the 
insulated  piece  C (fig.  88). 


Fig.  89. 


This  piece  is  commonly  made  of  copper  to  ensure  even 
wear  of  the  whole  commutator  cylinder,  but  as  it  is  insulated 


174  Electric  Lighting. 

and  not  connected  to  anything,  it  may  be  considered  as  an 
insulator. 

The  effect  of  it  is,  that  as  each  pair  of  coils  passes  the 
neutral  points,  and  the  E.M.F.  in  it  falls  to  zero,  that  pair 
of  coils  is  insulated  and  cut  out  of  circuit,  and  the  other  pair, 
which  is  in  quantity  with  it,  and  which  at  that  moment  has  its 
maximum  E.M.F. , works  alone.  This  entirely  gets  rid  of 
the  back  currents. 

A second  precisely  similar  pair  of  commutators  is  con- 
nected to  the  coils  2,  2 a,  4,  4a.  The  two  pairs  of  commu- 
tators, each  with  their  brushes,  are  seen  at  the  right  hand 
end  of  the  shaft  in  Plate  XXI. 

The  two  circuits  thus  formed  are  usually  connected  in 
series,  so  as  to  form  one  of  double  the  E.M.F.  of  each. 

The  magnets  are  also  connected  in  series  with  the  cir- 
cuits. Fig.  89  is  a general  diagram  of  the  connections. 

These  machines  are  very  successful  as  high-tension 
machines  for  arc-lighting,  as  machines  of  2000  volts 
E.M.F.,  maintaining  40  arcs  in  series,  are  in  regular  work. 

For  some  reason,  however,  the  inventor  has  not  been  suc- 
cessful in  making  low  pressure  machines  on  this  principle 
for  incandescent  lighting. 

II.  The  Siemens  Sub-Type. 

The  Siemens  Machine. 

The  revolving  portion  of  the  Siemens  machine  (fig.  90) 
consists  of  an  iron  cylinder,  round  which  wire  is  wound 
longitudinally , i.e.  so  that  the  wire  is  parallel  to  the  axis. 
The  collector  is  similar  to  that  of  machines  of  the  Gramme 
sub-type,  and  consists  of  a number  of  strips  of  metal  fixed 
on  an  insulating  barrel. 

The  wire  forms  a continuous  coil  or  closed  circuit,  and 
wires  are  led  sideways  from  it  at  intervals  to  the  commu- 
tator strips. 

The  magnets  are  made  of  bars  of  wrought-iron,  straight 
at  the  ends  and  curved  in  the  middle.  The  current  in  the 
magnetizing  coils  has  such  directions,  that  the  whole  of  the 
curved  portion  of  the  magnets  at  the  top  of  the  machine 


The  Siemens  Machine. 


175 


has  one  polarity,  and  that  at  the  bottom  of  the.  machine  the 
opposite.  The  outer  ends  of  the  upper  and  lower  magnets, 
which  are  of  opposite  polarities,  are  connected  by  yoke-plates 
in  the  usual  way. 

We  notice  that  the  machine  shown  in  fig.  90  is 


lifted  off  the  ground  by  legs.  The  reason  of  this  is, 
that  if  it  were  to  be  allowed  to  go  flat  on  the  ground, 
and  were  laid  on  an  iron  floor,  the  latter  would  make  a 
magnetic  connection  between  the  centre  and  ends  of  the 


90. 


i 76  Electric  Lighting. 

lower  magnets,  and  would  greatly  weaken  the  magnetism  of 
the  latter. 

These  machines  are  often  made  vertical,  i.e.  to  stand  on  one 
end.  They  then  occupy  less  space,  and  there  is  no  danger 
of  accidental  magnetic  communication  being  made  between 
their  poles. 

The  Large  Edison  Machine.  (Plate  XXII.) 

The  armature  of  this  machine  consists  of  a number  of  discs 
of  thin  iron  plate,  separated  by  paper  so  as  to  form  a barrel 
3 feet  6 inches  long.  A number  of  copper  rods  are  laid  on 
the  circumference  of  this  barrel,  parallel  to  its  axis.  The 
diameter  of  the  barrel  outside  the  bars  is  28 1 inches. 

It  is  necessary  to  connect  the  bars,  so  that  they  may  form 
a continuous  circuit  analogous  to  the  longitudinally-wound 
wire  in  the  Siemens  machine.  This  is  accomplished  by 
means  of  a number  of  copper  discs,  equal  to  the  number  of 
bars;  each  disc  has  two  lugs  projecting  from  it.  Half  the 
discs  are  put  at  each  end  of  the  barrel,  and  are  set  so  that 
the  lugs  form  in  each  case  a spiral  line  round  its  end  of  the 
barrel,  the  two  spirals  being  in  the  same  direction.  The  bars 
are  then  connected  from  lug  to  lug,  so  that  the  current  goes 
along  one  bar  across  a disc,  back  along  a bar  at  the  opposite 
side  of  the  barrel  to  the  first,  then  across  a disc  at  the  oppo- 
site end  of  the  barrel  to  the  first  disc,  and  then  along  the 
bar  next  to  the  first  bar,  and  so  on. 

The  copper  discs  are  connected  respectively  to  the  dif- 
ferent commutator  bars. 

The  whole  barrel  is  bound  round  and  round  with  steel 
wire  to  keep  the  bars  from  flying  out  under  the  action  of  the 
centrifugal  force. 

The  whole  barrel  revolves  between  the  poles  of  a very 
large  electro-magnet. 

These  poles  consist  of  immense  blocks  of  cast-iron, 
which  nearly  meet,  but  are  kept  apart  by  the  brass  distance- 
piece  seen  in  the  front  of  Plate  XXII. 

The  lines  of  force  leaving  the  magnets  terminate  in  tho 
central  iron  barrel.  The  construction  of  the  barrel,  i.e.  its 
being  built  up  of  iron  discs,  insulated  from  each  other,  pre- 


Plate  XXII. — the  edison  dynamo  machine. 


ENGINEERING  DEPARTMENT  LIBRARY 

WESTERN  LlEOTPiIC  COMPANY 
The  Large  Edison  Machine.  177 

vents  the  circulation  o£  currents  in  it.  The  armature  is 
kept  cool  by  means  of  a small  smith's  fan,  the  air  from 
which  is  admitted  by  the  pipes  seen  in  the  front  of 
Plate  XXII. 

The  magnet  bobbins  are  twelve  in  number,  and  are  each 
eight  feet  long.  They  are  shunt  wound. 

The  resistance  of  the  armature  is  *00049  ohm,  and  that 
of  the  magnets  21  ohms. 

The  machine  will  maintain  1000  to  1200  lamps  of  16 
candle-power.  Its  total  weight  is  about  25  tons. 

The  Edison  Company  also  make  small  machines  which 
work  well. 


N 


1 78 


Electric  Lighting. 


CHAPTER  XI Y. 

REGULATION  OF  MACHINES. 

If  tlie  number  of  lamps  on  a dynamo  machine  be  altered, 
the  electromotive  force  will  alter,  and  the  brightness  of  the 
lamps  will  also  alter.  Generally  speaking,  if  the  number  of 
lamps  be  diminished,  the  E.M.F.  will  increase,  and  vice  versa. 

This  change  of  E.M.E.  is  not  admissible  in  practical 
work,  and  has  to  be  corrected  by  various  methods. 

These  methods  differ  according  to  the  nature  and  size  of 
the  dynamo.  With  large  dynamos,  for  instance,  we  can 
afford  to  govern  by  hand,  as  the  wages  of  the  man  employed 
are  distributed  over  a very  large  number  of  lamps,  and 
hence  the  additional  cost  per  lamp  per  annum  caused  by 
his  wages  is  trifling. 

With  small  dynamos,  however,  the  man’s  wages  would  be 
distributed  over  only  a few  lamps,  and  hence  the  increased 
cost  per  lamp  per  annum  would  be  very  great. 

Eor  instance,  to  hand-govern  a machine  working  fourteen 
hours  daily  will  require  two  men,  whose  wages  together  would 
amount  to,  say,  £120  per  annum. 

Let  us  assume  that  the  rest  of  the  expense  amounts  to  £1 
per  lamp  per  annum,  then  with  a 5000-light  machine  these 
u , , 120  x 240 

wages  would  amount  to  — 5000  ~ per  lamp  per 

annum,  an  insignificant  addition  to  £1 ; but  if  we  have  only 
a 250-light  machine,  these  wages  per  lamp  per  annum  will 
be  115d.  = 9s.  7 d.  per  lamp  per  annum,  an  increase  of  nearly 
50  per  cent,  on  the  £1. 

Thus,  while  large  machines  may  be  hand-governed,  in 


Plate  XXIII.— apparatus  for  regulating  the  Gordon  dynamo. 


Regulation  of  the  Gordon  Dynamo.  179 

small  machines  it  is  necessary  to  put  up  with  much  un- 
steadiness in  the  light  in  order  to  use  automatic  methods 
of  governing,  none  of  which,  as  far  as  my  present  experience 
goes,  are  equal  to  hand-governing. 

The  following  is  the  method  I adopt  for  governing  my 
large  dynamos. 

It  will  be  remembered  that  the  large  machine  is  driven 
by  one  steam-engine,  and  the  “ exciting”  machine  by  a 
smaller  separate  one. 

A dark  room  (Plate  XXIII.)  is  provided  near  the  engines, 
and  the  steam-pipes  pass  through  it.  The  large  steam- 
pipe  supplies  steam  to  the  big  engine;  the  small  one 
supplies  it  to  the  exciting  engine. 

The  large  engine  has  an  ordinary  governor  on  it  of  the 
Porter  type,  and  the  large  stop-valve  is  opened  wide,  so  that 
the  governor  takes  charge  of  the  large  engine. 

The  small  engine  is  then  started  slowly,  and  the  lights 
begin  to  glow,  and  the  speed  is  increased  till  they  are  at 
their  right  candle-power,  as  shown  on  the  photometer*  at 
the  right  of  Plate  XXIII. 

In  case  of  a number  of  lamps  being  turned  off,  the  bright- 
ness of  the  rest  rises  somewhat,  and  the  man  in  charge 
slightly  closes  the  valve  of  the  small  engine,  reducing  its 
speed  until  the  light  is  right.  To  assist  him  in  making  a 
slight  motion,  a tangent  screw  is  attached  to  the  axis  of  the 
valve  wheel.  This  screw  can  be  instantly  thrown  out  of 
gear  on  removing  a wedge,  if  a large  motion  is  suddenly 
required. 

With  a large  system,  such  as  is  worked  by  one  of  these 
dynamos,  the  maximum  number  of  lamps  which  are  controlled 
by  any  one  switch  form  a very  small  percentage  of  the 
whole  number  at  work,  and  consequently  there  is  ample 
time  to  adjust  the  pressure  and  compensate  a change  of 
brightness  step  by  step,  so  that  it  never  reaches  an  amount 
which  can  be  seen  without  a photometer. 

One  dynamo  has  been  successfully  regulated  by  this 
method  for  fifteen  months. 


* See  Chapter  XVIII. 
N 2 


i8o 


E lectric  L ighting . 

On  the  right  of  Plate  XXIII.  is  seen  a steam  gauge  for 
showing  the  boiler  pressure,  and  on  the  left  is  a strophom  eter 
for  showing  the  speed  of  the  large  engine,  and  an  ammeter 
for  showing  the  strength  of  the  exciting  current.  The 
ammeter  is  ordinarily  short-circuited  by  a spring  key  seen 
below  it.  On  pressing  the  button  the  short-circuit  is  broken, 
and  the  whole  current  flows  through  the  ammeter. 

Automatic  Electric  G-overnors. 

Various  attempts  have  been,  and  are  being  made  to  do 
automatically  what  is  here  done  by  a man,  i.e.  to  vary  the 
speed,  either  of  the  exciting  engine  when  two  engines  are 
used,  or  of  the  main  engines  when  the  dynamo  is  self-excited, 
by  means  of  some  kind  of  voltmeter  actuating  the  throttle- 
valve,  so  that  just  as  an  ordinary  governor  keeps  the  speed 
constant,  so  the  electric  governor  would  vary  the  speed  so 
as  to  keep  the  E.M.F.  constant. 

The  Willans  Governor. 

The  only  one  of  the  various  governors  now  being  made 
which  I am  yet  able  to  describe  is  the  Willans  governor, 
and  even  this  is  so  lately  completed,  that  I can  give  no 
results  of  its  practical  working,  or  say  whether  it  or  any 
other  form  are  likely  to  be  a practical  success. 

The  following  description  of  the  apparatus  is  taken  from 
j Engineering  of  February  15,  1884: — 

“ The  difficulty,  hitherto,  has  been  to  get  a power 
sufficiently  large  to  be  independent  of  the  friction  of  the 
throttle  valve,  and  still  more,  of  that  of  the  expansion  valve, 
should  it  be  desired  to  govern  by  varying  the  expansion 
instead  of  by  throttling.  Mr  .Willans,  instead  of  actuating 
the  throttle  valve  or  expansion  valve  directly  by  the  electro- 
magnet or  solenoid,  employs  the  latter  to  actuate  a small 
supplementary  valve,  which  is  almost  frictionless,  and  this 
in  its  turn  controls  the  supply  or  discharge  of  water,  steam, 
or  other  fluid  pressure  to  a cylinder  in  which  a piston  works, 
which  actuates  the  throttle  valve  or  expansion  gear  of  the 
engine.  In  this  way,  although  absorbing  a power  less  than 


The  Willans  Electric  Governor. 


1 8 1 


half  that  required  for  one  20-candle  Swan  lamp,  the  solenoid 
is  able  to  control  the  most  powerful  expansion  gear. 

“ The  Willans  electric  governor 
is  shown  in  fig.  91,  where  S is  a 
solenoid  taking  the  place,  in  in- 
candescence lighting,  of  one  of 
the  lamps.  In  other  words,  the 
solenoid  is  on  a branch  between 
the  main  wires.  The  core  C of 
the  solenoid  is  suspended  by  a 
spring,  and  this  spring  is  attached 
at  the  top  to  an  adjusting  screw 
used  for  regulating  the  light. 

The  other  end  of  the  core  is  con- 
nected with  a small  piston  valve 
working  inside  the  main  piston 
W,  which  latter  piston  controls 
the  throttle  valve  in  thecasing  T. 

Water  or  other  fluid  pressure  is 
admitted  by  the  pipe  P into  an 
annular  chamber  surrounding  the 
water  piston  W,  and  also  by 
means  of  a suitable  passage  X, 
into  an  annular  space  between 
the  two  small  pistons  which  form 
the  piston  valve.  The  water, 
after  actuating  the  piston  W, 
escapes  through  the  pipe  E,  and 
by  means  of  a small  piece  of 
flexible  pipe  not  shown.  The 
action  is  as  follows  : — 

“ When  the  electromotive  force 
is  constant,  the  pull  of  the  spring 
balances  the  pull  of  the  solenoid 
coils  on  the  core  C,  but  if  the 
electromotive  force  rises,  on  ac- 
count either  of  an  increase  in 
steam  pressure  or  because  lights  Fig.  91. 

are  turned  out,  the  core  C is  drawn  further  into  the  coils  of 


=>j  ie?i— 


1 82  Electric  Lighting. 

the  solenoid,  and  moves  downwards,  carrying  with  it  the 
piston  valve.  The  latter  uncovers  the  port  A,  and  admits 
water  pressure  from  the  annular  space  between  its  pistons  to 
the  upper  side  of  the  water  piston  W,  which  then  travels 
downwards,  following  the  piston  valve.  So  soon  as  the 
electromotive  force  has,  by  the  closing  of  the  throttle  valve 
and  the  consequent  slowing  of  the  engine  and  dynamo, 
become  sufficiently  reduced,  the  core  C comes  to  rest,  and 
W consequently  overtakes  the  piston  valve,  and  closing  the 
port  A,  comes  also  to  rest.  When  the  electromotive  force 
falls  below  the  normal  standard,  the  foregoing  action  is,  of 
course,  reversed.  When  lights  are  turned  out  or  in  one  by 
one,  or  when  the  steam  pressure  rises  or  falls  gradually, 
the  action  of  the  governor  is,  of  course,  exceedingly  gradual, 
though  it  can  be  detected  by  measurement,  but  under  any 
violent  test,  such  as  switching  out  a large  proportion  of  the 
lights,  it  acts  with  great  quickness.  It  will  be  noticed 
how  the  perfect  action  of  such  a governor  is  helped  by  the 
ingenious  f differential  ’ movement  of  the  two  pistons,  and 
by  the  locking  action  of  the  piston  valve.” 

Compound  Winding.. 

Another  method  of  regulating,  but  which  can  only  be 
applied  to  direct-current  dynamos,  is  that  known  as  compound 
winding. 

The  E.M.F.  of  a “ series- wound  ” dynamo  increases  when 
more  lamps  are  put  on,  i.e.  when  the  external  resistance  is 
diminished,  but  the  E.M.F.  of  a shunt- wound  dynamo  de- 
creases under  the  same  circumstances. 

By  winding  the  magnets  partly  with  a thick  wire  connected 
in  series  with  the  armature,  and  partly  with  a thin  one 
connected  in  shunt,  it  is  possible,  within  certain  limits,  to  keep 
the  E.M.F.  nearly  constant,  in  spite  of  considerable  changes 
in  the  number  of  lamps  on  the  machine. 

The  proportion  between  the  shunt  and  series  wire  has  to 
be  found  experimentally  for  each  type  of  machine. 

The  first  approximation  is  made  by  taking  the  curves 
representing  the  respective  rise  and  fall  of  E.M.F.  with 


Compound  Winding.  183 

increased  number  of  lamps  for  shunt  and  series-wound  mag- 
nets. A straight  line  to  represent  constant  E.M.F.  being 
drawn  between  the  two  curves,  the  areas  on  each  side  of  it 
represent  respectively  the  weights  of  each  kind  of  wire 
required. 

In  spite  of  its  apparent  simplicity , I doubt  if  compound 
winding  will  be  much  used  in  the  future,  except  for  small 
machines,  as  it  does  not  keep  the  E.M.F.  quite  constant,  and 
the  apparatus  required  for  making  the  final  regulation  would 
equally  well  do  it  all. 


Conclusion. 

The  true  secret  of  successful  regulation  is  to  have  very 
large  dynamos , because  then,  as  we  have  said  before,  the 
maximum  number  of  lamps  that  can  be  turned  out  at  one 
time  is  a very  small  percentage  of  the  whole,  and  when  there 
are  a great  number  of  lamps  on  one  machine,  the  cost  per 
lamp  of  regulating,  either  by  hand  or  by  an  elaborate 
mechanical  contrivance,  is  very  trifling. 


184 


Elective  Lighting. 


CHAPTER  XV. 

ON  THE  PROPOSED  DISTRIBUTION  OP  ELECTRICITY  BY  SECONDARY 

GENERATORS. 

We  have  already  stated  (page  67)  that  the  quantity  of 
copper  required  to  convey  a certain  quantity  of  electrical 
energy  to  a given  distance  with  only  a certain  percentage 
loss  depends,  not  on  the  energy  but  only  on  one  factor  of 
it,  namely,  the  current,  and,  therefore,  if  with  our  given 
quantity  of  energy  we  can  increase  the  electromotive  force 
and  diminish  the  current,  we  can  use  less  copper. 

If,  for  instance,  a certain  weight  of  copper  is  required  to 
convey  one  electrical  H.P.  a certain  distance  with  a loss  of 
five  per  cent,  at  100  volts  pressure,  then,  if  the  pressure  is 
raised  to  1000  volts,  only  one-tenth  of  copper  will  be  required 
for  the  same  quantity  of  electrical  energy. 

Pressures  much  exceeding  100  volts  cannot,  however,  be 
used  for  incandescent  lamps  (as  at  present  constructed)  which 
are  required  to  be  turned  out  singly,  as  higher  pressures 
involve  putting  two  or  more  lamps  in  series. 

Further,  the  Board  of  Trade  have  very  rightly  forbidden 
the  use  in  indoor  wires  accessible  to  the  public  of  electricity 
at  pressures  exceeding  from  150  to  200  volts,  on  account  of 
the  danger  due  to  shocks  which  might  be  received  from  it. 

Various  attempts  have  been  made  to  convey  electricity  at 
high  pressures  from  the  generators  to  the  place  where  it  is 
to  be  used,  and  there  to  convert  it  into  low-pressure  electricity 
before  it  goes  to  the  lamps  or  other  fittings  inside  the  houses . 

The  Goulard  and  Gibbs  System. 

The  most  promising  system  for  this  purpose,  when  looked 


Goulard  and  Gibbs'  Secondary  Generators.  185 

at  superficially,  is  that  lately  invented  by  Messrs  Goulard  and 
Gibbs,  which  may  be  briefly  described  as  a system  of  inverted 
induction  coils : — 

The  ordinary  induction  coil*  consists  of  a coil  of  thick  wire 
with  an  iron  core,  surrounded  by  another  coil  of  fine  wire. 

A battery  current  of  low  pressure  is  sent  through  the 
thick  wire,  and  is  rendered  intermittent  by  a “ contact- 
breaker.”  At  each  intermittence  an  electromotive  force  is 
induced  in  each  of  the  convolutions  of  the  fine  wire.  As 
this  wire  has  a great  many  turns  and  a high  resistance,  the 
“ secondary  current  ” generated  will  have  small  quantity  but 
very  high  E.M.F.  With  the  largest  induction  coils  yet 
constructed,  pressures  of  a million  volts  and  over  have  been 
obtained. 

In  Nov.,  1879,  the  late  Mr.  William  Spottiswoode  pointed 
outf  that  if  the  primary  coil  is  excited  by  the  current 
of  an  alternating  machine  instead  of  by  a battery,  no  contact- 
breaker  is  required,  and  that  greatly  increased  results  are 
obtained. 

The  Goulard  and  Gibbs  apparatus  consists  essentially  of 
an  induction  coil,  of  which  the  'primary  coil  consists  of  a 
long  thin  wire,  and  the  secondary  of  a short  thick  one.  An 
alternating  current  of  small  quantity  but  of  high  pressure  is 
sent  into  the  primary,  and  induces  in  the  secondary  a current 
of  more  quantity  and  less  pressure,  which  can  be  used  in  the 
lamps.  Messrs.  Goulard  and  Gibbs*  scheme  is  to  place  such 
an  induction  coil  in  each  house,  or  group  of  houses,  and  to 
convey  the  electricity  from  the  generator  to  the  induction 
coil  in  the  form  of  a high-pressure  current,  which  can  be 
carried  by  a fine  wire,  and  so  to  save  copper. 

Efficiency. 

It  is  obvious  that  there  must  be  some  loss  in  this  as  in 
any  system  of  transformation.  In  particular,  the  whole  of 
the  energy  required  to  send  the  primary  and  secondary  cur- 
rents through  the  true  copper  resistances  of  the  primary  and 

# See  my  “ Electricity,”  2nd  ed.,  vol.  ii.,  page  107. 

t See  Phil.  Mag.,  1872,  page  360,  or  my  “ Electricity,”  2nd  ed.,  vol.  ii., 
page  124. 


i86 


E lectric  L igh  ting. 

secondary  coils  respectively  is  wasted  in  heating  these  coils. 
The  efficiency  E of  each  induction  coil  is  the  ratio  of  the 
electrical  energy  generated  in  the  secondary  to  that  ex- 
pended in  the  primary,  and  the  percentage  efficiency 
is : — 


Ep  = 100'  Ire! 


(59) 


where  C1}  C2  are  the  respective  currents  in  the  primary  and 
secondary  coils,  and  Et,  E2  the  respective  E.M.F.s  at  their 
terminals. 

Messrs  Goulard  and  Gibbs  have  not  yet  published  any 
figures  as  to  the  efficiency  of  their  coils.  Pending  their 
doing  so,  we  can,  however,  investigate  what  is  the  minimum 
value  it  must  have,  in  order  that  the  adoption  of  the  system 
may  reduce  the  first  cost  of  an  electric  light  plant. 

The  changes  in  the  different  items  of  first  cost  will  be  as 
follows : — 

The  total  weight  of  copper  wdll  be  reduced  in  the  ratio 
Ci 
C2* 

This  will  reduce  the  diameter  of  the  wires  in  the  ratio 


v/ 


C, 

<v 


The  thickness  of  the  insulator  must  be  increased  in  the 
E 

ratio  — 1 ; but,  as  the  wire  is  smaller,  the  weight  of  insulating 
E2 

material  will  be  increased  in  a less  ratio  than  this. 

The  total  cost  of  the  rest  of  the  plant,  i.e.  engines,  boilers, 

dynamos,  &c.,  will  be  increased  in  the  ratio  -g— * 

The  cost  will  also  be  increased  by  the  cost  of  the  induction 
coils  themselves. 

Let  us  investigate  a fairly  typical  case.  M e will  suppose 
we  have  an  ordinary  plant  costing  £20,000  arranged  to  supply 


* Por  suppose  we  lose  30  per  cent,  in  the  induction  coil,  or  that  EP  — 70, 
then  a plant  which  can  produce  1000  lights  direct  can  now  only  produce 

, . , 1000 

700,  and  to  make  it  do  1000  it  must  be  increased  in  the  ratio  “^qq  • 


i87 


Efficiency  of  Secondary  Generators. 

electricity  at  100  volts  pressure  ; let  us  consider  what  the 
minimum  efficency  EP  of  the  induction  coils  must  be,  in  order 
that  we  may  provide  plant  for  the  same  number  of  lights  for 
the  same  money  on  the  Goulard  and  Gibbs  system  with  the 
primary  E.M.F.  raised  to  1000  volts. 

In  the  ordinary  100-volt  system,  the  £20,000  may  be 
approximately  apportioned  as  follows  in  a moderately 


scattered  district : — 

Copper  .......  £5,000 

Insulator  ......  2,000 

Best  of  plant  (engines,  boilers,  dynamos, 

&c.) 13,000 


Total  . . £20,000 

On  increasing  the  E.M.F.  to  1000  volts,  we  alter  the  cost 
as  follows : — 

Copper £500 

Insulator,  say  . . . . . . 4,000 

Induction  coils,  say  .....  1,000 

Leaves  for  rest  of  plant  ....  14,500 


Total  . . £20,000 


In  order  that  the  plant  which  we  can  now  afford  may  be 
able  to  supply  the  required  quantity  of  electricity,  the 
efficiency  of  the  induction  coils  must  not  be  less  than — 

„ ™ 13,000  QO  , 

EP  = 100’  - = 89  per  cent. 

14,500  1 

I fear  that  it  is  not  likely  that  the  efficiency  will  be  any- 
thing like  so  high  as  this.  It  must  further  he  remembered 

that  the  hill  for  coals  will  he  increased  in  the  ratio  . 

EP 

It  would  be  necessary  further,  not  merely  that  the  plant 
should  cost  the  same  money,  but  that  there  should  be  a very 
great  economy,  in  order  to  compensate  for  the  extra  risk  run 
by  the  men  in  the  engine-room,  and  men  employed  in  street 
repairs,  who  might  accidentally  cat  or  break  the  primary 
wire,  and  who  if  they  did  so  would  probably  receive  a fatal 
shock. 

On  the  other  hand,  in  very  scattered  districts  (as,  for 


1 88  Electric  Lighting. 

instance,  the  stations  on  the  Metropolitan  Railway  which 
are  now  being  lighted  on  this  system  as  an  experiment), 
the  proportion  of  the  cost  of  the  copper  to  that  of  the  rest 
of  the  plant  in  a 100- volt  system  might  be  much  greater 
than  in  the  typical  case  which  we  have  suggested,  and  in 
such  a case  the  system  might  be  useful.  Each  case  where  it 
is  proposed  to  apply  the  system  should,  however,  be  inves- 
tigated by  the  method  given  above  and  discussed  on  its  own 
merits. 


189 


CHAPTER  XVI. 

THE  “ STORAGE  ” OP  ELECTRICITY SECONDARY  BATTERIES. 

The  advantages  obtained  by  the  storage  of  gas  in  gaso- 
meters have  suggested  to  many  inventors  the  hope  of 
storing  electricity,  or  rather  electric  energy,  in  a similar 
manner. 

The  only  way  in  which  electric  energy  could  be  stored 
directly  would  be  to  insulate  two  conductors  and  to  charge 
them  positively  and  negatively  respectively.  On  connecting 
them  by  a wire,  a current  would  How  from  one  to  the  other 
through  the  wire  until  the  pressures  were  equal.  This  wire 
might  be  interrupted  by  a lamp  through  which  the  current 
could  flow. 

The  method  is  of  course  impracticable  owing  to  the 
enormous  size  which  the  conductors  would  have  to  be  in 
order  to  hold  a charge  large  enough  to  produce  an  appre- 
ciable current  even  for  a short  time. 

The  two  conductors  may  be  arranged  to  form  the  plates 
of  an  ordinary  condenser  or  Leyden  jar,*  in  which  case  they 
will  hold  rather  more  energy  than  before,  but  still  not 
enough  to  be  of  any  practical  use. 

Seeing  then  that  the  direct  storage  of  electric  energy  is 
impracticable,  attention  was  called  to  the  storage  of  other 
kinds  of  potential  energy  in  a form  which  could  be  converted 
into  electric  energy  when  wanted.  This  potential  energy 
maybe  generated  in  various  forms,  and  the  energy  expended 
to  produce  it  may  be  either  electric  energy  or  energy  of 
some  other  kind. 

* See  my  “Electricity,”  2nd  ed.,  vol.  i.,  page  61. 


190 


E lectric  L ighting. 

For  instance,  the  potential  energy  which  we  are  storing 
ready  to  draw  out  in  the  form  of  electric  energy  may  be 
the  chemical  energy  latent  in  the  zinc  and  acid  of  an  ordi- 
nary voltaic  battery,  or  it  may  be  the  mechanical  and 
chemical  energy  respectively  of  the  steam  ready  under 
pressure  in  our  boiler,  and  of  the  coals  lying  ready  to  be 
shovelled  into  the  furnace. 

If  it  is  desired  to  produce  the  potential  energy,  which  is 
to  be  stored,  by  expending  electric  energy,  then  we  may  use 
a current  to  work  a motor  which  is  employed  in  compressing 
air  or  raising  water  to  a height,  so  that  the  air  or  water 
could  afterwards  work  a dynamo,  or  the  current  might  be 
employed  in  producing  chemical  charges  in  a secondary 

Secondary  Batteries. 

M.  Plante  has  found  that  if  two  sheets  of  lead  properly 
prepared!  be  placed  in  diluted  sulphuric  acid,  and  con- 
nected respectively  to  the  poles  of  a dynamo,  that  a chemical 
change  takes  place  in  them,  which  enables  them  to  act  as  a 
voltaic  battery  until  they  have  given  off  a quantity  of 
electric  energy  forming  a considerable  percentage  of  that 
expended  by  the  dynamo  in  “ charging  33  them. 

The  process  of  preparing  the  lead  being  a tedious  one, 
M.  Faure  invented  a battery  consisting  of  ordinary  plates  of 
lead  coated  with  “ nimium,”  or  red  oxide  of  lead ; and  in 
1880,  great  excitement  was  caused  in  England  by  an 
announcement  which  appeared  in  the  Times  that  “ a 
million  foot-pounds  of  electrical  energy  had  been  brought 
from  Paris  to  London  in  a small  portmanteau.” 

A million  of  anything  seems  to  be  a large  quantity,  but 
to  get  a true  idea  of  the  magnitude  of  a million  foot-pounds 
of  electrical  energy,  we  may  note  that  it  equals  *377  of  a 
commercial  unit,  and  at  the  maximum  price  authorized 
by  the  Board  of  Trade  for  the  St.  James's  district  is  worth 
2 '6dj  say  two-pence  halfpenny. 

An  immense  number  of  modifications  and  improvements 
of  the  secondary  battery  have  been  patented  since  the  above 

t See  my  “ Electricity,”  2nd  ed.,  vol.  ii.,  page  10. 


Secondary  Batteries.  191 

date,  but  I have  not  as  yet  seen  one  which  has  worked  with 
even  reasonable  success. 

Even  when  new  and  freshly  charged  the  percentage 
return  is  not  very  large,  not  more  than  about  75  per  cent, 
at  most,  i.e.  the  energy  given  out  is  not  more  than  75  per 
cent,  of  that  expended  in  “ charging”  the  batteries. 

Secondly,  the  batteries  will  not  hold  a “"charge”  for 
any  length  of  time.  I mean  that,  if  charged  and  put 
away  for  a week,  the  return  at  the  end  of  the  week  is 
much  less  than  with  a battery  freshly  charged.  This  loss 
is  due  to  local  chemical  actions  taking  place  inside  the 
batteries. 

Thirdly,  the  batteries  rapidly  wear  out,  and  after  a few 
month s'  work  require  new  lead  plates. 

Fourthly,  their  first  cost  per  unit  of  electrical  energy  which 
they  can  store  (in  the  form  of  chemical  energy)  is  very  heavy. 

There  is  no  doubt  that  the  interest  and  depreciation  on  a 
set  of  secondary  batteries  large  enough  to  enable  an  electric 
light  plant  to  worh  day  and  night,  and  so  give  out  to  the 
lamps  no  electricity  in  the  day  but  a double  quantity  in  the- 
night,  is  vastly  greater  than  the  interest  and  depreciation  on  a 
complete  duplicate  set  of  engines,  boilers,  and  dynamos. 

The  more  we  consider  the  question  of  the  storage  of 
electrical  energy,  the  more  we  shall  be  convinced  that  the 
best  form  of  potential  energy  in  which  to  keep  it  is  in  that 
of  the  potential  energy  of  coals  and  compressed  steam,  and 
the  proper  place  in  which  to  store  it  is  a spare  boiler  kept 
ready  to  actuate  a spare  engine  and  dynamo. 

The  potential  energy  contained  in  a battery  rapidly  leaks 
out.  Boilers  do  not  leak  at  all.  Engines  are  comparatively 
cheap,  and  last  indefinitely ; batteries  are  dear,  and  wear  out 
rapidly.  The  only  storage  apparatus  which  is  worthy  the 
name  is  a spare  boiler  full  of  steam,  with  a banked  fire,  and 
a spare  engine  and  dynamo,  kept  warm,  well  oiled,  and 
ready  to  start  at  a moment's  notice. 

Storage  of  High-pressure  Currents. 

There  is  one  form  in  which  the  proposed  use  of  secondary 
batteries  will  deserve  consideration  when  a practical  secon- 


192 


Electric  Lighting . 


dary  battery  shall  have  been  constructed.  It  has  been 
proposed  that  the  engines  and  dynamos  shall  be  placed 
outside  the  town  to  be  lighted , and  that  the  batteries  shall 
be  kept  in  the  centre  of  the  town.  It  is  then  proposed  that 
a high-pressure  current  shall  be  brought  from  the  dynamos 
to  the  batteries  by  the  use  of  fine  wire,  a great  number  of 
the  batteries  being  arranged  “ in  series  33  to  receive  the 
charge,  and  then  altered  to  a u quantity 33  arrangement  to 
discharge  to  the  lamps. 

The  relative  economic  advantages  of  this,  and  of  a direct 
supply,  may  be  calculated  by  the  method  given  in  the  last 
chapter  as  applied  to  the  Goulard  and  Gibbs  system. 


193 


CHAPTER  XVII. 


EXPERIMENTAL  MEASUREMENTS  OF  HORSE-POWER. 


The  liorse-power  being  developed  at  any  moment  by  each 
cylinder  of  an  engine  is  given  by  the  formula — 


where — 


H.P. 


2SRAP 

33,000 


(60) 


S = Length  of  stroke  in  feet. 

R = Number  of  revolutions  per  minute. 

A = Area  of  piston  in  square  inches. 

P = Mean  pressure  on  the  piston  during  the  whole  stroke  in 
pounds  per  square  inch. 

It  will  be  noted  on  examining  this  formula  that  2SR  is 
the  speed  in  feet  per  minute  at  which  the  piston  moves;  and 
AP  is  the  mean  total  weight  in  pounds  pressing  on  the  piston. 

2SRAP  is  therefore  the  number  of  foot-pounds  per 
minute  which  is  being  expended. 

One  H.P.  is  equal  to  33,000  foot-pounds  per  minute,  and 
hence  the  number  of  H.P.  is  equal  to  the  number  of  foot- 
pounds per  minute  divided  by  33,000,  which  gives  the 
formula  (60)  above. 

For  any  given  engine  the  quantities  S and  A are  con- 
stant, R is  kept  constant  by  the  governor,  and  P constantly 
varies,  for  as  the  load  is  changed,  as  for  instance,  by  the 
turning  on  or  shutting  off  of  lamps,  the  throttle  valve  (or 
expansion  valve,  as  the  case  may  be)  is  more  or  less  opened 
or  shut  by  the  action  of  the  governor.  When  the  valve  is 
more  nearly  closed,  P of  course  diminishes. 

All  indicators  are  instruments  for  measuring  P (the 
mean  pressure  in  the  cylinder)  at  any  instant. 


o 


194 


Electric  Lighting. 

Eichards’  Indicator. 

Eichards’  indicator  (Fig.  91)  is  an  instrument  of  this  de- 


Fig.  91. 

soription.  It  consists  of  a small  steam  cylinder  with  a piston 
in  it.  The  cylinder  is  connected  by  a pipe  to  the  cylinder 


195 


Indicators  : Richards — Boys . 

of  the  engine,  and  is  driven  up  by  the  steam  against  a 
spring  whi  ch  tends  to  force  it  down.  The  height  at  which 
the  small  piston  stands  at  any  instant,  indicates  the  pres- 
sure in  the  engine-cylinder  at  that  instant.  This  pressure 
varies  from  zero  at  one  part  of  the  stroke  to  its  maximum 
value  at  another  part.  In  order  to  obtain  its  mean  value 
during  the  whole  stroke,  a pencil  is  attached  to  the  rod  of 
the  indicator-piston,  which,  as  the  piston  moves,  would 
draw  a vertical  line  on  a stationary  piece  of  paper. 
The  maximum  height  of  this  line  would  represent  the  maxi- 
mum steam  pressure  during  the  stroke.  The  paper  is, 
however,  not  stationary,  but  is  wound  round  a barrel  which 
is  connected  by  a string  to  the  piston-rod  of  the  engine  so 
that  it  revolves  backwards  and  forwards  on  its  axis,  making 
about  three-quarters  of  a revolution  for  each  stroke.  If 
the  indicator- piston  is  at  rest,  the  pencil  will  trace  on  the 
paper  a horizontal  line  round  the  barrel.  When  the  indi- 
cator-piston and  paper  both  move,  a curved  line  will  be 
traced  by  the  pencil,  whose  vertical  height  above  any  point 
of  the  horizontal  zero  line  gives  the  pressure  in  the  cylinder 
at  the  portion  of  the  stroke  represented  by  that  point. 

The  total  area  of  the  space  included  between  the  curve  and 
the  horizontal  line , divided  by  the  length  of  the  horizontal  line , 
gives  the  mean  pressure  throughout  the  stroke,  and  this  is  the 
quantity  P which  we  want  to  know. 

I do  not  propose  to  give  any  directions  for  the  practical 
use  of  the  indicator,  as  it  is  well  understood  by  engineers, 
and  whenever  an  engine  is  erected,  there  will  always  be 
some  one  in  charge  who  will  know  how  to  indicate  it. 

Boys'  Engine-power  Meter. 

Mr.  Vernon  Boys  has  devised  an  instrument  for  indicating 
the  mean  pressure  in  an  engine- cylinder,  which  consists  of  a 
cylinder,  piston,  and  spring,  the  communication  with  which 
from  the  engine-cylinder  is  made  by  a long,  fine  pipe,  through 
which  the  steam  cannot  move  rapidly.  Instead  of  oscillating- 
up  and  down  during  the  stroke,  the  friction  of  the  steam 
causes  the  piston  to  remain  stationary  in  the  position  indi- 
cating the  mean  pressure.  I have  had  no  practical  ex- 

o 2 


196 


Electric  Lighting . 

perience  of  this  instrument,  but  it  may  probably  be  very 
useful  as  a continuous  indicator  for  the  engines  of  a central 
station.  It  can  also  be  made  self-recording. 

Maximum  Horse-Power. 

The  maximum  value  of  P,  the  mean  pressure  which  an 
engine  working  at  a fairly  economical  expension  rate  can  have, 
may  be  taken  as  at  about  half  the  boiler  pressure,  and  there- 
fore if  we  wish  to  find  the  maximum  horse-power  which  we 
are  likely  to  get  out  of  a given  engine,  we  may  use  the 
formula  (60),  and  take  P as  half  the  boiler  pressure. 

All  the  above  calculations  apply  to  simple  engines  with 
one  cylinder.  When  two  cylinders  are  used,  the  horse- 
power will  of  course  be  doubled.  The  method  of  obtaining 
the  H.P.  of  compound  engines  is  more  complex,  and  I shall 
not  touch  upon  it. 

Horse-Power  given  off  by  a Shaft. 

With  small  experimental  machines  it  is  convenient  some- 


Fig.  92. 

times  to  measure  the  horse-power  which  is  being  transmitted 


197 


Horse- Power  given  off  by  a Shaft. 

by  a shaft  or  coupling.  Fig.  92  represents  an  apparatus  for 
that  purpose  devised  by  Professor  Ayrton.  The  coupling 
between  the  driving  and  receiving  shaft  is  not  rigid,  but  is 
made  by  a spring.  The  receiving  shaft  therefore  lags  be- 
hind the  other  by  an  amount  depending  on  the  force  applied 
to  it.  This  lagging  causes  the  white  lever  to  move  in 
towards  the  shaft,  and  the  circle  described  by  the  bright  bead 
at  its  end  is  diminished  in  diameter.  The  diameter  of  this 
circle  gives  the  pounds  of  pull.  The  velocity  of  the  shaft 
gives  the  feet  per  minute,  and  the  product  of  the  two  is  the 
foot-pounds  per  minute,  or  33,000  times  the  horse-power. 


Fig.  93. 


Fig.  93  is  another  form  of  the  same  apparatus  arranged  for 
measuring  the  H.P.  transmitted  by  a belt. 

Speed. 

Young's  Speed  Indicator. 

Fig.  94  represents  an  apparatus  by  which  the  speed  of 
any  shaft,  of  which  the  end  is  accessible,  can  be  at  once 
noted  without  timing.  On  the  little  point  being  pressed 
into  the  hollow  at  the  end  of  the  shaft,  the  hand  at  once 
points  to  the  number  of  revolutions  per  minute. 


198 


Electric  Lighting . 

The  apparatus  consists  of  a miniature  governor  like  an 
engine-governor,  but  controlled  by  a spring  instead  of  by  a 
weight.  As  the  balls  fly  out  they  move  a lever,  which  moves 
the  hand. 


The  point  can  be  put  on  either  of  the  two  spindles  shown, 
one  being  used  for  speeds  under  500  revolutions  per 
minute,  and  indicating  on  the  inner  circle  on  the  dial,  and 
the  other  for  speeds  up  to  2000  revolutions,  and  indicating 
on  the  outer  circle. 


199 


CHAPTER  XVIII. 


PHOTOMETRY. 


It  is  very  important  to  be  able  to  measure  accurately  the 
candle-power  of  incandescent  lamps. 

The  simplest  photometer  known  is  one  of  the  best  in 
practice. 

It  consists  of  a rod  or  wire  placed  about  1J  inches  in 
front  of  a white  screen.  The  two  lights  which  are  to  be 
compared,  as  for  instance,  an  incandescent  lamp  and  a 
standard  candle,  are  placed  at  different  distances  from  the 
screen,  and  the  distance  of  one  varied  until  the  two  shadows 
of  the  wTire  thrown  on  the  screen  are  of  equal  tint. 

The  relative  intensities  of  the  lights  are  then  inversely  as 
the  squares  of  their  distances  from  the  screen . 

In  practice,  the  electric  lamp  is  placed  at  a fixed  distance 
of  say  100  inches  from  the  screen,  and  the  candle  is  shifted 
till  the  shadows  are  equal.  Then  if  DL  is  the  distance  of  the 
lamp  from  the  screen,  and  Dc  that  of  the  candle,  the  candle- 
power  K is — 


To  save  working  out  this  equation  for  every  experiment,  the 
distances  Dc  of  the  candle  corresponding  to  different  candle- 
powers  K are  worked  out  by  the  following  formula, — 


• (61) 


VK 


'l 


. (62) 


They  are  then  marked  on  the  board,  and  their  corresponding 


200 


E lectric  L igh  ting . 

candle-powers  marked  on  them.  To  take  an  observation, 
the  candle  is  then  slid  along  till  the  shadows  are  equal,  and 
the  candle-power  read  off  from  the  position  of  the  candle  on 
the  board. 

“ Standard  candles  99  can  be  obtained  of  Messrs.  Sugg,  at 
Charing  Cross. 

This  firm  also  make  a very  elaborate  photometer,  which 
they  supply  to  gas-works,  and  which  is  suitable  to  large 
electric-light  factories.  It  is  unnecessary  to  occupy  space 
by  describing  it  here,  as  the  details  of  it  are  only  of  interest 
to  those  who  have  to  use  it,  and  they  will  be  understood  from 
the  directions  sent  with  it. 


201 


CHAPTER  XIX. 

CENTRAL  STATION  LIGHTING. 

I had  intended  to  write  a long  chapter  with  the  above  head- 
ing, but,  for  various  reasons,  I am  not  yet  prepared  to  do 
so.  I have,  however,  left  in  the  heading  for  the  convenience 
of  inserting  such  a chapter  in  a future  edition  of  this  book, 
should  one  ever  be  required. 


202 


Electric  Lighting . 


CHAPTER  XX. 

METERS. 

There  have  been  a great  number  of  patents  taken  out  for 
electric-meters,  i.e.  for  instruments  to  continuously  register 
tbe  number  of  commercial  units  of  electric  energy  used  by 
each  consumer. 

None  of  them,  however,  as  yet  work  satisfactorily.  There 
is  no  doubt  that  good  meters . will  be  forthcoming  when 
wanted,  i.e.  when  district  lighting  under  this  year’s  Pro- 
visional Orders  is  commenced  next  year.  At  present  there 
has  been  no  demand  for  them,  and  inventors  have  been  busy 
at  other  things. 

The  three  principal  types  of  meter  now  in  existence  in  a 
crude  form  are  Edison’s,  Hopkinson’s,  and  my  own. 

Edison’s  consists  of  two  plates  of  copper  in  an  electrolytic 
cell,  through  which  a small  known  fraction  of  the  current 
passes.  The  total  energy  consumed  is  then  measured  by 
weighing  the  plates,  and  noting  the  respective  loss  and 
gain  from  month  to  month. 

Hopkinson’s  meter  consists  of  a small  electric  motor, 
which  revolves  at  varying  speed  according  to  the  strength 
of  current,  and  which  records  the  total  number  of  revolutions 
taken  per  month. 

My  own  meter  consists  of  some  kind  of  galvanometer, 
carrying  an  eccentric  or  :c  snail  ” wheel,  so  arranged  that 
the  vertical  radius  gets  less  as  the  current  gets  stronger. 
At  short  intervals  an  arm,  lifted  by  clockwork,  drops  from 
a fixed  zero  position  to  the  edge  of  the  snail.  The  distance 
through  which  it  moves  is  greater  as  the  current  is  greater. 
The  apparatus  records  the  total  length  of  all  the  journeys 
taken  by  the  arm  in  a month  or  quarter,  in  its  searches  for 
the  galvanometer-wheel. 


203 


CHAPTER  XXI. 


FIRE  RISKS. 

The  risk  of  fire  from  electric  light  apparatus  is  very  small, 
but  fires  may  occur  through  the  carelessness  or  ignorance 
of  the  engineers  who  erect  the  plant.  In  order  to  minimize 
this  risk,  the  Society  of  Telegraph  Engineers  have  drawn 
up  the  following  regulations  : — 


RULES  AND  REGULATIONS 

FOR  THE  PREVENTION  OF  FIRE  RISKS  ARISING  FROM  ELECTRIC 

LIGHTING, 


Recommended  by  the  Council  of  the  Society  of  Telegraph  Engineers 
and  of  Electricians,  in  accordance  with  the  Report  of  the  Committee 
appointed  by  them  on  May  11,  1882,  to  consider  the  subject. 

MEMBERS  OF  THE  COMMITTEE. 


Professor  W.  G.  Adams,  F.R.S.,  Vice- 
President. 

Sir  Charles  T.  Bright. 

T.  Russell  Crampton. 

R.  E.  Crompton. 

W.  Crookes,  F.R.S. 

Warren  De  la  Rue,  D.C.L.,  F.R.S. 
Professor  G.  C.  Foster,  F.R.S.,  Past 
President. 

Edward  Graves. 

J.  E.  H.  Gordon. 

Dr.  J.  Hopkinson,  F.R.S. 


Professor  D.  E.  Hughes,  F.R.S.,  Vice- 
President. 

W.  H.  Preece,  F.R.S.,  Past-Presi- 
dent. 

Alexander  Siemens. 

C.  E.  Spagnoletti,  Vice-President. 

James  N.  Shoolbred. 

Augustus  Stroh. 

Sir  William  Thomson,  F.R.S.,  Past 
pres  id 

Lieut.-Colonel  C.  E.  Webber,  R.E., 
Past  President. 


These  rules  and  regulations  are  drawn  up  for  the  re- 
duction to  a minimum,  in  the  case  of  electric  lighting,  of 
those  risks  of  fire  which  are  inherent  in  every  system  of 
artificial  illumination,  and  also  for  the  guidance  and  in- 
struction of  those  who  have,  or  who  contemplate  having, 
electric  lighting  apparatus  installed  in  their  premises. 


204  Electric  Lighting . 

The  difficulties  that  beset  the  electrical  engineer  are 
chiefly  internal  and  invisible,  and  they  can  only  be  effectually 
guarded  against  by  “ testing,”  or  probing  with  electric 
currents.  They  depend  chiefly  on  leakage,  undue  resist- 
ance in  the  conductor,  and  bad  joints,  which  lead  to  waste 
of  energy  and  the  dangerous  production  of  heat.  These 
defects  can  only  be  detected  by  measuring,  by  means  of 
special  apparatus,  the  currents  that  are  either  ordinarily 
or  for  the  purpose  of  testing,  passed  through  the  circuit. 
Should  wires  become  perceptibly  warmed  by  the  ordinary 
current,  it  is  an  indication  that  they  are  too  small  for  the 
work  they  have  to  do,  and  that  they  should  be  replaced  by 
larger  wires.  Bare  or  exposed  conductors  should  always 
be  within  visual  inspection,  and  as  far  out  of  reach  as 
possible,  since  the  accidental  falling  onto,  or  the  thoughtless 
placing  of  other  conducting  bodies  upon  such  conductors 
would  lead  to  “ short  circuiting,”  and  the  consequent 
sudden  generation  of  heat  due  to  an  increased  current 
in  conductors  not  adapted  to  carry  it  with  safety. 

The  necessity  cannot  be  too  strongly  urged  for  guarding 
against  the  presence  of  moisture  and  the  use  of  (i  earth  ” as 
part  of  the  circuit.  Moisture  leads  to  loss  of  current  and 
to  the  destruction  of  the  conductor  by  electrolytic  corrosion, 
and  the  injudicious  use  of  “ earth  ” as  a part  of  the  circuit 
tends  to  magnify  every  other  source  of  difficulty  and 
danger. 

The  chief  dangers  of  every  new  application  of  electricity 
arise  from  ignorance  and  inexperience  on  the  part  of  those 
who  supply  and  fit  up  the  requisite  plant. 

The  greatest  element  of  safety  is  therefore  the  employ- 
ment of  skilled  and  experienced  electricians  to  supervise 
the  work. 

I.  The  Dynamo  Machine. 

1.  The  dynamo  machine  should  be  fixed  in  a dry  place. 

2.  It  should  not  be  exposed  to  dust  or  flyings. 

3.  It  should  be  kept  perfectly  clean  and  its  bearings  well 
oiled. 

4.  The  insulation  of  its  coils  and  conductors  should  be 
practically  perfect. 


Fire  Risks. 


205 


5.  All  conductors  in  the  dynamo -rooiia  should  be  firmly 
supported^  well  insulated,  conveniently  arranged  for  in- 
spection, and  marked  or  numbered. 

II.  The  Wires. 

6.  Every  switch  or  commutator  used  for  turning  the 
current  on  or  off  should  be  constructed  so  that  when  it  is 
moved  and  left  it  cannot  permit  of  a permanent  arc  or  of 
heating. 

7.  Every  part  of  the  circuit  should  be  so  determined, 
that  the  gauge  of  wire  to  be  used  is  properly  proportioned 
to  the  currents  it  will  have  to  carry,  and  all  junctions  with 
a smaller  conductor  should  be  fitted  with  a suitable  safety 
fuse  or  protector,  so  that  no  portion  of  the  conductor  should 
ever  be  allowed  to  attain  a temperature  exceeding  150°  F. 

8.  Under  ordinary  circumstances  complete  metallic  cir- 
cuits should  be  used;  the  employment  of  gas  or  water 
pipes  as  conductors  for  the  purpose  of  completing  the 
circuit,  should  not  in  any  case  be  allowed. 

9.  Bare  wires  passing  over  the  tops  of  houses  should 
never  be  less  than  seven  feet  clear  of  any  part  of  the  roof, 
and  all  wires  crossing  thoroughfares  should  invariably  be 
high  enough  to  allow  fire  escapes  to  pass  under  them. 

10.  It  is  most  essential  that  joints  should  be  electrically 
and  mechanically  perfect  and  united  by  solder, 

11.  The  position  of  wires  when  underground  should  be 
clearly  indicated,  and  they  should  be  laid  down  so  as  to  be 
easily  inspected  and  repaired. 

12.  All  wires  used  for  indoor  purposes  should  be  efficiently 
insulated,  either  by  being  covered  throughout  with  some 
insulating  medium,  or,  if  bare,  by  resting  on  insulated 
supports. 

13.  When  these  wires  pass  through  roofs,  floors,  walls, 
or  partitions,  or  where  they  cross  or  are  liable  to  touch 
metallic  masses,  like  iron  girders  or  pipes,  they  should  be 
thoroughly  protected  by  suitable  additional  covering ; and 
where  they  are  liable  to  abrasion  from  any  cause,  or  to  the 
depredations  of  rats  or  mice,  they  should  be  efficiently 
encased  in  some  hard  material. 


206  Electric  Lighting. 

14.  Where  indoor  wires  are  put  out  of  sight,  as  beneath 
flooring,  they  should  be  thoroughly  protected  from  mechani- 
cal injury,  and  their  position  should  be  indicated. 

N.B. — The  value  of  frequently  testing  the  apparatus  and 
circuits  cannot  be  too  strongly  urged.  The  escape  of 
electricity  cannot  be  detected  by  the  sense  of  smell,  as  can 
gas,  but  it  can  be  detected  by  apparatus  far  more  certain 
and  delicate.  Leakage  not  only  means  waste,  but  in  the 
presence  of  moisture  it  means  destruction  of  the  conductor 
and  its  insulating  covering,  by  electrolytic  action. 

III.  Lamps. 

15.  Arc  lamps  should  always  be  guarded  by  proper 
lanterns  to  prevent  danger  from  falling  incandescent  pieces 
of  carbon,  and  from  ascending  sparks.  Their  globes  should 
be  protected  with  wire  netting. 

16.  The  lanterns,  and  all  parts  which  are  to  be  handled, 
should  be  insulated  from  the  circuit. 


The  “ safety-fuse  ” spoken  of  in  § 7 consists  of  a short 
length  of  wire  of  lead  or  other  fusible  metal  inserted  in  the 
circuit,  the  diameter  being  such  that,  if  the  current  increases 
say  50  per  cent,  above  its  proper  value,  the  fusible  wire  will 
melt  and  break  the  circuit. 


APPENDIX. 


WEIGHTS  AND  RESISTANCES  OF  PURE  COPPER  WIRES. 


208 


A ppendix. 


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MHOO(MlCQOH0COGOlO(MHO5CSO3O'!flH'fHCOOHCOlOGO»OlOCDNQOM®OJMi>CO 
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2 12 


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<DCQ 


Appendix, 


213 


__  _ Tfi^(Mi>co^(NONWi>sqo 

40  rH  05  00  00  t>»  05  00  C ^ i>»  05  H*  © H*  rH  ^PH050505000505HW 

^odoi>cbvb^<ttOjo3rH©©d5a5dodoi>n^j>cbcbcbci 
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IM(MOOOONQOTjl0COH(MiOO^COOQOCDTflNCO(jOl>NroO5OHN©^HOCOHHiOH 
HH05^rHOOOOCOCOHIVOOO©©Cpppi;~pppprHpa005qOTHrHTfipiHp<>4t>'H<ppp 
C00303d3rHrHrHrHrHrHrHrH03Wd303C0'^^vbcbl>d0d005O03d0rtflbj^d0OrHiro'^cbd0 
<MCO^lOCO^QOOiOH(Mm^iOCDI>QOmOH(MCO^kOCDOOG30HNCO-^®J>QOOlOH 
J>J>l>t>i>M>l>Q000  (»Q0(»Q0a)00(»000J  05  05  a05  05  05  03  aOOOOOOOOOHH 

i — I r— I 1— I 1— 1 I — I 1— I r- 1 1— I H H H 

t^rH05O5Tfia0rHTjlvOC0i>.VO00©C0H''Hia5C0vOI>05vOvOvO05e003O03'HIC5VOH<rJI00  03  a5 
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HfflNWmH0500?OTflCOH0500CO>nm(M005X©lO'fNH005N©lO'fmN005(»I> 
«5^^i(T}('JmC005C0C0«lNlMNIMNtM(MHHHHHHHHOOOOOOOOClQ05 
i—l  r— I rH  i—l  1— I 1— I i—l  H H rH  1— I r— I 1— I 1— I 1— I 1— I 1— I 1— I rH  rH  1— I 1— I 1— I 1— I 1— I 1— I 1— I 1— I rH  1— I 1— I 1— I 1— I rH  1— I O O © 

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 

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W^CDOJNiOOlMlXNCOlNNCOOOiOHOOlOiMai^riliMHCniOOCDlOiJ^mCOCOMWOT^ 

COCDif(NHa)l>5D^COHOlX)t>«3^03HOO!NOl0^05HOaiQOt>tt)»0'l«NHOa) 

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1— I i—l  i—l  i—l  i—l  rH  i—l  rH  r— I i—l  i—l  i—l  i—l  rH  1— I 1— I 1— I 1— I 1— I i—l  i—l  1— I i—l  rH  i—l  1— I 1— I O O O O O O O O O O O 

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cooi>cooauo(MaiOi)iHaicomHaj5o^(MONko«)Hait 
i}i^MC0MN(NNHrlHHpopp9999900(»00lXli>l; 
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(MOOONJ>Q003(MtOOOHQOCO»OOCOt>OmNM05<X)'^N(MCCT}H>g-f05iO(NOOOCO 


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65  o 03  co  vh>  i>  60 


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laOS^OOmNiMOHCOOlOOiOOl^OilCB^ai^O^ai^O^OlOOiOiHOHtsNCO 
MCO^^OlOOON^OOWOOOpOHHWiNnn^ilipiOCpN^QOCOaiOpOHH 


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, ^ >!■  w T-i  ji — ii«>i*vJH(30tDH(MH05  0t)5DUJi}IMWIMHHHOOHH 

W-a>MH0505OHH00N00K5(MHHNWOOiOH^i0HH»nNO^C3lOiMO0503O 

OOHt>W00'^lHNC005tDiMOHD05ON’i}(H05®',!tl^rBl^lOfftHriCintQiOHm-inr) 

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214 


Appendix. 


10 


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2I5 


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A ppendtx. 


NATURAL  SINES,  &c. 


Deg. 

Sine. 

Cover. 

Cosec. 

Tan. 

Cotan. 

Secant. 

Yrsn. 

Cosin . 

0 

00 

1 -ooooo 

Infinite. 

•o 

Infinite. 

1-00000 

0 

1-00000 

90 

1 

01745 

•98254 

57-2986 

•01745 

57-2899 

100015 

0001 

•99984 

89 

2 

03489 

•96510 

28-6537 

•03492 

28-6362 

L -00060 

0006 

•99939 

88 

3 

05233 

•94766 

191073 

•05240 

190811 

1-00137 

0013 

•99862 

87 

4 

06975 

•93024 

14*3355 

•06992 

14-3006 

1-00244 

0024 

•99756 

86 

5 

08715 

•91284 

11-4737 

•08748 

11-4300 

1-00381 

0038 

•99619 

85 

6 

10452 

•89547 

9-5667 

•10510 

9-5143 

1-00550 

0054 

•99452 

84 

7 

12186 

•87813 

8-2055 

•12278 

81443 

1-00750 

0074 

•99254 

83 

8 

13917 

•86082 

7-1852 

•14054 

7-1153 

1-00982 

0097 

•99026 

82 

9 

15643 

•84356 

6-3924 

•15838 

6-3137 

1-01246 

0123 

•98768 

81 

10 

17364 

•82635 

5-7587 

•17632 

5-6712 

1-01542 

0151 

■98480 

80 

11 

19080 

•80019 

5-2408 

•19438 

51445 

1-01871 

0183 

•98162 

79 

]2 

20791 

•79208 

4-8097 

•21255 

4-7046 

1-02234 

0218 

•97814 

78 

13 

22495 

•77504 

4-4454 

•23086 

4-3314 

1-02630 

0256 

•97437 

77 

14 

24192 

•75807 

41335 

•24932 

40107 

1-03061 

0297 

•97029 

76 

15 

25881 

•74118 

3-8637 

•26794 

3-7320 

1-03527 

•0340 

•96592 

75 

16 

27563 

•72436 

3-6279 

•28674 

3-4874 

1-04029 

0387 

•96126 

74 

17 

•29237 

*70762 

3-4203 

•30573 

3-2708 

1-04569 

•0436 

•95630 

73 

18 

•30901 

•69098 

3-2360 

•32491 

30776 

1-05146 

■0489 

•95105 

72 

19 

•32556 

•67443 

30715 

•34432 

2-9042 

105762 

0544 

•94551 

71 

20 

•34202 

•65797 

29238 

•36397 

2-7474 

1-06417 

•0603 

•93969 

70 

,1 

•35836 

•64163 

2-7904 

•38386 

2-6050 

1-07114 

•0664 

•93358 

69 

22 

•37460 

•62539 

2 6694 

•40402 

2-4750 

1-07853 

•0728 

•92718 

68 

23 

•39073 

•60926 

2-5593 

•42447 

2*3558 

1-08636 

•0794 

•92050 

67 

24 

•40673 

•59326 

2-4585 

•44522 

2-2460 

1-09463 

•0864 

•91354 

66 

25 

•42261 

•57738 

2-3662 

•46630 

21445 

1-10337 

•0936 

•90630 

65 

26 

•43837 

•56162 

2-2811 

•48773 

20503 

1-11260 

•1012 

•89879 

64 

27 

•45399 

‘54600 

2*2026 

•50952 

1-9626 

1-12232 

•1089 

•89100 

63 

28 

•46947 

•53052 

2-1300 

•53170 

1-8807 

1 13257 

•1170 

•88294 

62 

29 

•48480 

•51519 

2-0626 

•55430 

1-8040 

1 14335 

1253 

•87461 

61 

30 

•50000 

•50000 

: 2-0000 

' 57735 

1-7320 

1-15470 

•1339 

•86602 

60 

1 31 

•51503 

•48496 

1-9416 

i -60086 

i 1-6642 

1-16663 

1428 

•85716 

59 

| 32 

•52991 

•47008 

1-8870 

' -62486 

1-6003 

117917 

1519 

•84804 

58 

! 33 

•54463 

•45536 

1-8360 

> -64940 

1-5398 

1 1*19236 

1613 

•83867 

57 

34 

•55919 

1 *44080 

' 1-7882 

•67450 

i 1-4825 

1-20621 

•1709 

•82903 

56 

35 

•57357 

•42642 

1-7434 

! *70020 

i 1-4281 

1-22077 

•1808 

•81915 

55 

36 

•58778 

i -41221 

1-7013 

! *72654 

1-3763 

1-23606 

1909 

•80901 

54 

37 

■60181 

•33818 

1-6616 

1 -75355 

, 1-3270 

1 1-25213 

•2013 

•79863 

53 

38 

•61566 

1 -38433 

1-6242 

•7812b 

, 1-2799 

' 1-26901 

•2119 

•78801 

52 

39 

•62932 

: -37067 

1-5890 

> -80978 

, 1-2348 

1-28675 

•2228 

•77714 

51 

40 

•64278 

i -35721 

1-5557 

•83909 

' 1-1917 

1-30540 

•2339 

•76664 

50 

41 

•65605 

•34394 

1-5242 

i -86928 

: 1-1503 

1-32501 

•2452 

•75470 

49 

42 

•66913 

: -33086 

1-4944 

•90040 

i 1-1106 

1-34563 

•2568 

•74314 

48 

43 

•68199 

' 31800 

1-4662 

: -93251 

1-0723 

1-36732 

•2686 

•73135 

47 

44 

•69465 

. -30534 

1-4395 

•96568 

1-0355 

1-39016 

•2806 

•71933 

46 

45 

•70710 

i -29289 

1-4142 

1-00000 

1 1-0000 

1-41421 

•2928 

•70710 

45 

j 

Cosin. 

Yersin. 

Secant. 

Cotan. 

Tan 

Cosec. 

Covn. 

Sine. 

Deg. 

AREAS  OF  CIRCLES,  ADVANCING  BY  IOths. 


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CM  CM  CM  CM  CO 


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254-469 

283-529 

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9 

Diam. 

© rH  CM  CO  Tp  to 

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AREAS  OF  CIRCLES,  ADVANCING  BY  IOths— Continued. 


218 


A ppendix 


Diara. 

H N 

CM  CM  OM  CM  CM 

CD  l>  00  05  O 
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CO  CO  CO  CO  CO 

CD  M>  X 05  O 
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05 

376-685 

411-871 

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486-955 

526-854 

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10  co  co  r> 

799-230 

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1188-41 

125036 

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05 

op 

373-253 

408-282 

441-881 

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522-793 

564-105 

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05  HH  05  ID  O 
M>  00  00  05  rH 

1063-62 

1122-21 

1182-37 

1244-10 

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369-837 

404-708 

411151 

479-164 

518-748 

559-903 

602-629 

646-926 

692-793 

740-231 

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839-820 

891-970 

945-692 

1000-98 

1057-84 

1116-28 

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1237*86 

1301-00 

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cp 

366-436 

401-150 

437436 

475-292 

514-719 

555-717 

598-286 

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363-051 
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101062 
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1158-11 
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CO 

356-328 

390-571 

426-385 

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CD  M>  X 05  O 
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| 

AREAS  OF  CIRCLES,  ADVANCING  BY  IOtiis—  Continued. 


A ppendix.  219 


1 Diam. 

r-l  M 05  tP  «3 
rp  rp  rp  rp  rp 

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00  »b  00  CO  dp 
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to  CD  |T  00  05 

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cq  05  wo  cq  05 

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05  cq  it  cb  rH 

CD  rp  rT  C5  It 

CD  It  oo  00  05 

rH  rH  rH  rH  rH 

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to  CO  rH  C5  OO 
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cq  cq  cq  cq  cq 

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IT  CD  to  rp  CO 
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rH 

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OHCD-f  O 

05  05  tD  It  tp 

cq  cq  oo  cq  co 
00  |T  rH  cq  p 

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cq  00  wo  cq  05 
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cq  do  cb  © to 

rp  cq  O 05  It 

O — i cq  cq  co 
cq  cq  cq  cq  oq 

do  dp  di  do  It 
CO  to  rp  co  cq 
rp  to  CO  It  GO 

cq  cq  cq  cq  cq 

Diam. 

rH  Oq  00  rH  10 
rji  rr  rp  rp  rP 

CD  It  00  05  O 
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WO  to  to  W5  WO 

CO  It  oo  05  O 
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AKEAS  OF  CIRCLES,  ADVANCING  BY  IOths—  Continued. 


220 


Appendix. 


a 

oS 

fi 

rH  <M  CO  Tfl  US 
CO  CO  CO  CO  CO 

CONOOOSO 
CO  CO  CO  CO  £>• 

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00 

os 

3009-34 

310736 

3206-95 

330811 

3410-84 

3515-14 

3621-01 

3728-45 

3837-47 

3948-05 

4060-21 
! 4173-93 
4289*23 
4406-10 
4524-54 

4644-54 

4766-12 

4889-27 

5014-00 

5140-29 

05 

00 

2999-63 

3097-49 

3196-92 

3297*92 

3400-49 

3504-64 

3610-35 

371764 

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3936-92 

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00  03  Ol 

5 CO  OS  rH 

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4632-47 

4753-96 

4876-89 

5001-44 

512759 

op 

2989-93 

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3287-75 

3390-17 

3494-16 
3599-71 
3706-84 
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3925-81 

403765 

4151-06 

4266-04 

4382-60 

4500*72 

4620-42 

4741-68 

4864-52 

4988-93 

5114-90 

l> 

cp 

2980-24 
3077-79 
3176-91 
3277  59 
337985 

3483-68 

3589-08 

3696-06 

3804-60 

3914-71 

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4139-65 

4254-48 

4370-87 

4488-84 

4608-38 

4729-49 

4852-16 

4976-42 

5102-24 

CD 

Areas. 

US 

297057 

3067-96 

3166-92 

3267-46 

3369-56 

3473-23 

3578-47 

3685-29 

3793-67 

3903-63 

4015-16 

4128-25 

4242-92 

4359-16 

4476-97 

4596-35 

4717-30 

4839-83 

4963*92 

5089-58 

V 

2960-92 

305815 

3156-96 

325733 

3359-28 

3462-79 

3567-8S 

3674-54 

3782-76 

3892-56 

4003-93 

4116-87 

4231-38 

4347-47 

4465-12 

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4705-14 

4827-50 

4951-44 

5076-95 

CO 

2951-28 

3048-36 

314701 

3247-22 

334901 

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us  us  cd  r>  oo 

tJH  us  CO  £■»  00 
CO  CO  CO  OS  CO 

3992-73 
4105-51 
4219-86 
4335-79 
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4572-35 

4692-99 

4815-20 

4938-98 

5064.32 

co 

2941-66 
3038-58 
313707 
! 3237  13 

j 333876 

3441-96 

3546-74 

3653-08 

3760-99 

3870-48 

3981-53 
4094  16 
4208-36 
4324*12 
4441-46 

4560-37 

4680-85 

4802-90 

4926-53 

5051-72 

N 

rH 

CO  US  US  CO 

O <X)  rH  O US 

cq  oo  i>- do 

CO  CM  <M  <M  <M 
OS  O 1— 1 05  00 
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3431-57 

353619 

3642-37 

3750-13 

3859-46 

3970-36 

4082-83 

4196-87 

4312-48 

4429-66 

4548-41 

4668-73 

4790-63 

4914-09 

5039-13 

rH 

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2922-47 

3019-07 

311725 

3216-99 

3318-31 

3421-20 

3525-66 

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3959-20 
4071  51 
4185-39 
4300-85 
4417-87 

4536-47 

4656-63 

4778-37 

4901-68 

5026-56 

p 

Diam. 

h cq  co  io 
co  CO  CO  CO  CO 

co  i>  oo  cs  O 

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AREAS  OF  CIRCLES,  ADVANCING  BY  IOths  - Continued. 


Appendix . 


221 


Diam. 

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rH  03  X Tp  xo 
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I'M 

222  A ppendix. 


STRENGTH  AND  WEIGHT  OF  METALS. 


Specific 

Gravity. 

Weight  of 
a cubic 
foot. 

Weight  of 
a cubic 
inch. 

Tensile 
Strength 
per  sq.  in. 

Crushing 
Weight 
per  sq.  in. 

lbs. 

lbs. 

tons. 

tons. 

Aluminium,  sheet  . . 

2-67 

166-6 

•096 

— 

— 

,,  „ cast  . . 

2-56 

159-8 

•092 

— 

— 

Antimony,  cast 

672 

419-5 

•242 

•47 

— 

Bismuth,  cast 

9822 

6131 

*353 

1-45 

— 

Copper  holts  . . 

8-85 

552-4 

•318 

17 

— 

,,  cast  .. 

8-607 

5373 

•31 

8-4 

— 

„ sheet . . 

8*78 

548-1 

*316 

13*4 

— 

,,  wire  . . 

8-9 

555 

•32 

26 

— 

Gold  

19-361 

1208-5 

•697 

9-1 

— 

Iron,  cast,  from 

7 

437 

•252 

6 

36 

„ „ to..  .. 

7-6 

474-4 

•273 

13 

64 

,,  ,,  average  . 

7*23 

451 

•26 

7-3 

48 

„ wrought,  from 

7-6 

474-4 

•273 

16 

16 

„ „ to  . . 

7-8 

486-9 

•281 

29 

18 

„ „ average 

7-78 

485 "6 

•28 

22 

16-9 

,,  wire 

— 

— 

— 

40 

— 

Lead,  cast 

11*36 

708-5 

•408 

•8 

3-1 

,,  sheet  .. 

11-4 

711-6 

•41 

1*5 

— 

Mercury 

13-596 

848-75 

•48945 

— 

— 

Platinum 

21-531 

1343-9 

*775 

— 

— 

,,  sheet 

23 

1435-6 

•828 

— 

— - 

Silver  

10-474 

653*8 

1 -377 

18-2 

— 

Steel  

8 

499 

•288 

52 

150 

„ plates 

_ 

— 

— 

35 

90 

Tin,  cast 

7-291 

4551 

•262 

2-0 

6 7 

Zinc,  cast 

7 

437 

•252 

33 

— 

tons. 


2 

3*4 

2*6 

3 

5-5 

3-8 


Transverse 

Strength. 


AIR. 


223 


— coar. 


INDEX. 

A. 

Air-pump,  Lane-Fox’s,  82. 

Steam’s,  65. 

Alternate -current  machine,  phases  of,  134. 

currents,  measurement  of,  60. 

Ammeter,  Ayrton’s,  44. 

Ampere,  the,  defined,  10. 

Ampere’s  theory  of  magnetism,  7. 

Arc  lamps,  84. 

principle  of,  9. 

Areas  of  circles,  217. 

Armature  coils,  148. 

Artificial  lighting,  principles  of,  1. 

Ayrton  and  Perry’s  ammeter,  44. 

• electric-power- meter,  58. 

ohmmeter,  56. 

voltmeter,  50. 

Ayrton’s  mechanical  horse-power  indicator,  196. 

B. 

Battery,  conventional  sign  for,  25. 

Board  of  Trade  unit,  20. 

Boys’  engine-power  meter,  195. 

Brush’s  dynamo-machine,  169. 

lamp,  94. 

Burgin  dynamo,  the,  167. 

c. 

Carbons  for  arc  lamps,  100. 

Cardew’s  voltmeter,  52. 

Central  station  lighting,  201. 

Centrifugal  force,  150. 

Circles,  areas  of,  217. 

Coefficient  of  self-induction,  125. 

mutual  induction,  121. 

Commercial  unit,  20. 


Index . 


COM 


224 

Compound  winding,  182. 

■ wound  dynamos,  142. 

Conductors  defined,  5. 

Continuity  of  physical  change,  111. 

Copper,  current,  and  E.M.F.,  67. 

wires,  weights  and  resistances  of,  208. 

Core,  iron,  116. 

Coulomb,  the,  defined,  12. 

Crompton-Burgin  dynamo,  the,  167. 
Crompton’s  lamp,  87. 

Current,  measurement,  of,  35. 

unit  of,  10. 

Currents,  electric,  conversion  of,  into  heat,  4. 


D. 

De  Meritens  magneto  machine,  156. 

Direct-current  machines,  general  types  of,  137,  167. 
Divided  circuits,  22. 

Dynamo  machines,  the  designing  of,  145. 

machine,  the  Brush,  169. 

the  Crompton-Burgin,  167. 

— Edison’s,  176. 

Ferranti’s,  160. 

• Gordon’s,  162. 

Siemens’  alternating,  159. 

Siemens’  direct-current,  174. 

Dynamometer,  Siemens’  electro-,  35. 

E. 

Edison’s  dynamo-machine,  176. 

lamp,  73. 

Efficiency  and  durability  of  incandescent  lamps,  74. 
Electric  currents,  conversion  of,  into  heat,  4. 

generators,  theory  of,  113. 

principles  of,  130. 

Electro-magnetic  induction,  111. 

magnets,  construction  of,  117. 

motive  force,  measurement  of,  49. 

unit  of,  10. 

Energy,  rate  of  expenditure  of,  13. 


F. 


Factor  of  safety,  152. 

Ferranti’s  dynamo-machine,  160. 


Index. 


225 


JAB. 


Foot-pound  defined,  13. 

Formulae  about  electric  units,  19. 
Field,  magnetic,  110. 

magnets,  magnetization  of,  141. 

Fire  risks,  203. 

Friction  of  water  produces  heat,  4. 

G. 


Galvanometers,  principle  of,  37- 
Galvanometer  shunts,  39. 

tangent,  40. 

Thomson’s  graded,  47. 

reflecting,  38. 

Gas  and  electricity,  units  of,  compared,  21. 
Gordon’s  dynamo-machine,  162. 

magnetic-field  measurer,  117. 

Goulard  and  Gibbs’  system,  184. 

Gramme  machine,  theory  of,  138. 

sub-type  of  dynamo,  167. 


H. 

Horse-power  defined,  13. 

electric,  measurement  of,  58. 

experimental  measurements  of  mechanical,  193. 

maximum  of  an  engine,  196. 

pull,  151. 

wasted  in  conductors,  26. 


I. 

Incandescent  lamps,  61 . 

principle  of,  9. 

Incandescence  of  solids  produces  light,  3. 
Indicator,  Boys’,  195. 

Eichards’,  194. 

Induction,  coefficient  of  mutual,  121. 

self-,  123. 

coefficient  of,  125. 

electro-magnetic,  111. 

magnetic,  110. 

Insulators  defined,  5. 

Iron  core,  116. 


Jablochkoep  candle,  99. 


j. 


2 26 


Index. 


LAM. 


L. 

Lamps,  see  Incandescent,  Arc,  &c. 

arc  and  incandescent,  principles  of,  9. 

Lane-Fox’s  air-pump,  82. 

furnace,  79. 

lamp,  78. 

Lenz’s  law,  115. 

Lime- light,  2. 

Lines  of  magnetic  force,  109. 

M. 

Magnets,  108. 

Magnetic  induction,  1 10. 

field,  110. 

measurement  of,  117. 

Magnet,  motion  of,  past  coil,  133. 

Magneto-machine,  De  Meritens,  156. 

Mathematical  analysis,  application  of,  to  machine  construction,  143. 
Maxim’s  lamp,  75. 

Measurement  of  magnetic  field,  117. 

Metals,  strength  and  weight  of,  222. 

Meters,  202. 

Miner’s  lamp,  Swan’s,  72. 

Motion  of  magnet  past  coil,  133. 

Multiple  arc,  22 . 

Mutual  induction,  coefficient  of,  121. 

o. 

Ohm’s  law,  12. 

Ohm,  the,  defined,  11. 

Ohmmeter,  Ayrton  and  Perry’s,  56. 


p. 


Parallel  circuit,  22. 

Phases  of  an  alternate-current  machine,  131. 
Photometry,  199. 

Platinum,  light  from,  when  heated,  8. 
Polarity  and  direction  of  current,  108. 
Pressure,  unit  of,  10. 

Principles  of  electric  generators,  130. 


Q- 


Quantity,  connection  in,  22. 
unit  of,  12. 


Index. 


227 


TEM. 

R. 

Reaction  of  armature  coil  on  magnets,  135. 
Regulation  of  machines,  178. 

Resistance,  measurement  of,  52. 

with  strong  currents,  55. 

unit  of,  11. 

Richards’  indicator,  194. 


s. 

Saturation,  117. 

Secondary  batteries,  189. 

generators,  184. 

Self  adjustment  of  current  by  lamps,  26. 

induction,  123. 

co-efficient  of,  125. 

effect  of,  in  dynamo-machines,  137. 

Series,  connection  in,  22. 

wound  dynamos,  142. 

Serrin’s  lamp,  86. 

Shunts  for  galvanometer,  25,  39. 

Shunt-wound  dynamos,  142. 

Siemens’  alternating-machine,  159. 

direct-current  dynamo-machine,  174. 

■ — electro-dynamometer,  35. 

Sines,  tangents,  &c.,  216. 

Speed  indicator,  Young’s,  197. 

of  dynamos,  153. 

Sprengel  air-pump,  65. 

Steam’s  air-pump,  65. 

Storage  batteries,  189. 

Strength  and  weight  of  metals,  222. 

Swan’s  lamp,  new  pattern,  67. 

old  pattern,  63. 


T. 

Table  of  areas  of  circles,  217. 

carbons  for  arc  lamps,  106,  107. 

efficiency  of  Edison  lamps,  74. 

■ Swan  lamps,  70. 

strength  and  weight  of  metals,  222. 

sines,  tangents,  &c.,  216. 

weights  and  resistances  of  pure  copper  wires,  208. 

Tangent  galvanometer,  40. 

Tangents,  sines,  &c.,  216. 

Temperature  scale  for  incandescent  lamps,  74. 


228 


Index. 


THE 


Theory  of  direct-current  machines,  138. 

electric  generators,  113. 

the  Gramme  machine,  138. 

Siemens  machine,  14<1. 

Thomson’s  graded  galvanometer,  47. 

— — — voltmeter,  30. 

u. 

Unit,  the  commercial,  20. 

Units,  10. 

V. 

Variation  of  current,  effect  of,  115. 

Volt,  the,  defined,  10. 

Voltmeter,  Ayrton  and  Perry’s,  50. 

— Cardew’s,  52. 

— Thomson’s,  50. 


w. 

Water,  analogies  between  the  flow  of,  and  of  currents,  4. 
Willans’  electric  governor,  180, 

Wires,  weights  and  resistances  of  copper,  208. 
Wheatstone’s  bridge,  52. 

Work,  unit  of,  13. 

Y. 

Young’s  speed  indicator,  197. 


THE  END. 


c\ 


CULBEBT  AND  BIVINGTON,  LIMITED,  ST.  JOHN’S  SQUABE,  LONDON. 


